Reference signals design for time tracking in LTE-A

ABSTRACT

Time tracking in current communication systems may be traditionally based on common reference signals (CRS). However, in certain communication systems, CRS-based time tracking may be impossible to implement due to an absence of CRS in certain subframes or carriers. CRS-based time tracking may also be inappropriate to implement in certain communication systems such as a coordinated multipoint (CoMP) system where control and data may arrive from different cells, and therefore, a UE may assume a wrong cell for CRS-based time tracking. Accordingly, methods, apparatuses, and computer program products for wireless communication are provided in which additional UE specific reference signals (UE-RS) and/or channel state information reference signals (CSI-RS) are made available to the UE so that the UE may have improved channel estimation and/or time tracking performance.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application Ser.No. 61/579,428, entitled “IMPROVED REFERENCE SIGNALS DESIGN FOR TIMETRACKING IN LTE-A” and filed on Dec. 22, 2011, U.S. ProvisionalApplication Ser. No. 61/600,190, entitled “IMPROVED REFERENCE SIGNALSDESIGN FOR TIME TRACKING IN LTE-A” and filed on Feb. 17, 2012, and U.S.Provisional Application Ser. No. 61/625,577, entitled “IMPROVEDREFERENCE SIGNALS DESIGN FOR TIME TRACKING IN LTE-A” and filed on Apr.17, 2012, and which are expressly incorporated by reference herein intheir entireties.

BACKGROUND

1. Field

The present disclosure relates generally to communication systems, andmore particularly, to methods for and apparatuses with improvedreference signals design for time tracking in Long Term Evolution (LTE)Advanced (LTE-A).

2. Background

Wireless communication systems are widely deployed to provide varioustelecommunication services such as telephony, video, data, messaging,and broadcasts. Typical wireless communication systems may employmultiple-access technologies capable of supporting communication withmultiple users by sharing available system resources (e.g., bandwidth,transmit power). Examples of such multiple-access technologies includecode division multiple access (CDMA) systems, time division multipleaccess (TDMA) systems, frequency division multiple access (FDMA)systems, orthogonal frequency division multiple access (OFDMA) systems,single-carrier frequency divisional multiple access (SC-FDMA) systems,and time division synchronous code division multiple access (TD-SCDMA)systems.

These multiple access technologies have been adopted in varioustelecommunication standards to provide a common protocol that enablesdifferent wireless devices to communicate on a municipal, national,regional, and even global level. An example of an emergingtelecommunication standard is LTE. LTE is a set of enhancements to theUniversal Mobile Telecommunications System (UMTS) mobile standardpromulgated by Third Generation Partnership Project (3GPP). It isdesigned to better support mobile broadband Internet access by improvingspectral efficiency, lower costs, improve services, make use of newspectrum, and better integrate with other open standards using OFDMA onthe downlink (DL), SC-FDMA on the uplink (UL), and multiple-inputmultiple-output (MIMO) antenna technology. However, as the demand formobile broadband access continues to increase, there exists a need forfurther improvements in LTE technology. Preferably, these improvementsshould be applicable to other multi-access technologies and thetelecommunication standards that employ these technologies.

SUMMARY

Time tracking in current communication systems may be traditionallybased on common reference signals (CRS). However, in certaincommunication systems, CRS-based time tracking may be impossible orinappropriate to implement. Accordingly, methods for and apparatuseswith an improved reference signals design are provided infra. Themethods/apparatuses allow a user equipment (UE) to utilize received userequipment specific reference signals (UE-RS) and/or channel stateinformation reference signals (CSI-RS) for improved time tracking.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be an evolvedNode B (eNB), restricts a number of resource blocks that can beallocated to a UE in a downlink assignment to be greater than or equalto two. In addition, the apparatus transmits a downlink transmissioncorresponding to the downlink assignment to the UE.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be a UE,receives a plurality of resource blocks in a transmission. The pluralityof resource blocks includes a precoding resource block group (PRG). Theapparatus decodes UE-RS based on an assumed same precoding fortransmission of the resource blocks in the PRG. The apparatus performstime tracking based on the decoded UE-RS in the PRG.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be a UE,receives at least one resource block in a transmission. Each of the atleast one resource block includes a first set of UE-RS. The apparatusdetermines whether a resource block of the at least one resource blockincludes a second set of UE-RS. The apparatus performs time trackingbased on the first set of UE-RS and based on the second set of UE-RSwhen the resource block is determined to include the second set ofUE-RS.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be an eNB,configures a UE to receive one of a rank one transmission or a rank twotransmission. The apparatus transmits a resource block to the UE. Theresource block includes a first set of UE-RS and a second set of UE-RS.One of the first set of UE-RS and the second set of UE-RS is for the UE,the other one of the first set of UE-RS and the second set of UE-RS isfor another UE or no other UEs.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be a UE,receives a configuration to receive at least one resource block with afirst number of CSI-RS ports in each resource block of the at least oneresource block. The apparatus receives the at least one resource blockin a transmission. The apparatus assumes a resource block of the atleast one resource block includes a second number of CSI-RS portsgreater than the first number of CSI-RS ports. The apparatus performstime tracking based on signals in resource elements corresponding to theassumed second number of CSI-RS ports.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be an eNB,configures a UE to receive a first number of CSI-RS ports. The apparatustransmits to the UE a resource block including a second number of CSI-RSports greater than the first number of CSI-RS ports. The second numberof CSI-RS ports enables improved time tracking by the UE.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be a UE,receives UE-RS and CSI-RS in at least one resource block. The UEperforms time tracking based on the received UE-RS and CSI-RS.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be a UE,receives at least one resource blocks in a transmission, each of the atleast one resource blocks comprising a first group of UE-referencesignals (UE-RS) associated with a first antenna port. The apparatusdetermines whether the at least one resource blocks comprise a secondgroup of UE-RS associated with one or more other antenna ports, andprocesses the received at least one resource blocks based on the firstgroup of UE-RS, and further based on the second group of UE-RS when theat least one resource blocks is determined to comprise the second groupof UE-RS.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be a UE,receives at least one resource block in a transmission, where the atleast one resource block comprises a first set of reference signals(RS), specific to the UE, determines whether a second set of RS,specific to the UE, is available in the transmission, and processes thereceived at least one resource block based on the first set of RS andfurther based on the second set of RS if determined available.

In an aspect of the disclosure, a method, a computer program product,and an apparatus are provided. The apparatus, which may be an eNB,configures a user equipment (UE) to receive a transmission, transmits tothe UE at least one resource block in the transmission, where the atleast one resource block comprises a first set of reference signals(RS), specific to the UE, and provides a second set of RS in thetransmission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a network architecture.

FIG. 2 is a diagram illustrating an example of an access network.

FIG. 3 is a diagram illustrating an example of a DL frame structure inLTE.

FIG. 4 is a diagram illustrating an example of an UL frame structure inLTE.

FIG. 5 is a diagram illustrating an example of a radio protocolarchitecture for the user and control planes.

FIG. 6 is a diagram illustrating an example of an evolved Node B anduser equipment in an access network.

FIG. 7 is a diagram illustrating the location of UE-RS within varioustypes of subframes and for describing exemplary methods for UE-RS-basedtime tracking.

FIG. 8 illustrates diagrams of reference signal configurations within aset of resource blocks and for describing exemplary methods forCSI-RS-based time tracking.

FIG. 9 is a diagram for illustrating the exemplary methods.

FIG. 10 is a diagram illustrating positions of UE-RS signals receivedvia antenna ports 7 and 8 in a subframe.

FIG. 11 is a flow chart of a method of wireless communication.

FIG. 12 is a flow chart of a method of wireless communication.

FIG. 13 is a flow chart of a method of wireless communication.

FIG. 14 is a flow chart of a method of wireless communication.

FIG. 15 is a flow chart of a method of wireless communication.

FIG. 16 is a flow chart of a method of wireless communication.

FIG. 17 is a flow chart of a method of wireless communication.

FIG. 18 is a flow chart of a method of wireless communication.

FIG. 19 is a flow chart of a method of wireless communication.

FIG. 20 is a conceptual data flow diagram illustrating the data flowbetween different modules/means/components in an exemplary apparatus.

FIG. 21 is a conceptual data flow diagram illustrating the data flowbetween different modules/means/components in an exemplary apparatus.

FIG. 22 is a diagram illustrating an example of a hardwareimplementation for an apparatus employing a processing system.

FIG. 23 is a diagram illustrating an example of a hardwareimplementation for an apparatus employing a processing system.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known structures and components areshown in block diagram form in order to avoid obscuring such concepts.

Several aspects of telecommunication systems will now be presented withreference to various apparatus and methods. These apparatus and methodswill be described in the following detailed description and illustratedin the accompanying drawings by various blocks, modules, components,circuits, steps, processes, algorithms, etc. (collectively referred toas “elements”). These elements may be implemented using electronichardware, computer software, or any combination thereof. Whether suchelements are implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem.

By way of example, an element, or any portion of an element, or anycombination of elements may be implemented with a “processing system”that includes one or more processors. Examples of processors includemicroprocessors, microcontrollers, digital signal processors (DSPs),field programmable gate arrays (FPGAs), programmable logic devices(PLDs), state machines, gated logic, discrete hardware circuits, andother suitable hardware configured to perform the various functionalitydescribed throughout this disclosure. One or more processors in theprocessing system may execute software. Software shall be construedbroadly to mean instructions, instruction sets, code, code segments,program code, programs, subprograms, software modules, applications,software applications, software packages, routines, subroutines,objects, executables, threads of execution, procedures, functions, etc.,whether referred to as software, firmware, middleware, microcode,hardware description language, or otherwise.

Accordingly, in one or more exemplary embodiments, the functionsdescribed may be implemented in hardware, software, firmware, or anycombination thereof. If implemented in software, the functions may bestored on or encoded as one or more instructions or code on acomputer-readable medium. Computer-readable media includes computerstorage media. Storage media may be any available media that can beaccessed by a computer. By way of example, and not limitation, suchcomputer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or otheroptical disk storage, magnetic disk storage or other magnetic storagedevices, cloud/network storage, or any other medium that can be used tocarry or store desired program code in the form of instructions or datastructures and that can be accessed by a computer. Disk and disc, asused herein, includes compact disc (CD), laser disc, optical disc,digital versatile disc (DVD), floppy disk and Blu-ray disc where disksusually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

FIG. 1 is a diagram illustrating an LTE network architecture 100. TheLTE network architecture 100 may be referred to as an Evolved PacketSystem (EPS) 100. The EPS 100 may include one or more user equipment(UE) 102, an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN)104, an Evolved Packet Core (EPC) 110, a Home Subscriber Server (HSS)120, and an Operator's IP Services 122. The EPS can interconnect withother access networks, but for simplicity those entities/interfaces arenot shown. As shown, the EPS provides packet-switched services, however,as those skilled in the art will readily appreciate, the variousconcepts presented throughout this disclosure may be extended tonetworks providing circuit-switched services.

The E-UTRAN includes the evolved Node B (eNB) 106 and other eNBs 108.The eNB 106 provides user and control planes protocol terminationstoward the UE 102. The eNB 106 may be connected to the other eNBs 108via an X2 interface (e.g., backhaul). The eNB 106 may also be referredto as a base station, a base transceiver station, a radio base station,a radio transceiver, a transceiver function, a basic service set (BSS),an extended service set (ESS), or some other suitable terminology. TheeNB 106 provides an access point to the EPC 110 for a UE 102. Examplesof UEs 102 include a cellular phone, a smart phone, a session initiationprotocol (SIP) phone, a laptop, a personal digital assistant (PDA), asatellite radio, a global positioning system, a multimedia device, avideo device, a digital audio player (e.g., MP3 player), a camera, agame console, or any other similar functioning device. The UE 102 mayalso be referred to by those skilled in the art as a mobile station, asubscriber station, a mobile unit, a subscriber unit, a wireless unit, aremote unit, a mobile device, a wireless device, a wirelesscommunications device, a remote device, a mobile subscriber station, anaccess terminal, a mobile terminal, a wireless terminal, a remoteterminal, a handset, a user agent, a mobile client, a client, or someother suitable terminology.

The eNB 106 is connected by an S1 interface to the EPC 110. The EPC 110includes a Mobility Management Entity (MME) 112, other MMEs 114, aServing Gateway 116, and a Packet Data Network (PDN) Gateway 118. TheMME 112 is the control node that processes the signaling between the UE102 and the EPC 110. Generally, the MME 112 provides bearer andconnection management. All user IP packets are transferred through theServing Gateway 116, which itself is connected to the PDN Gateway 118.The PDN Gateway 118 provides UE IP address allocation as well as otherfunctions. The PDN Gateway 118 is connected to the Operator's IPServices 122. The Operator's IP Services 122 may include the Internet,the Intranet, an IP Multimedia Subsystem (IMS), and a PS StreamingService (PSS).

FIG. 2 is a diagram illustrating an example of an access network 200 inan LTE network architecture. In this example, the access network 200 isdivided into a number of cellular regions (cells) 202. One or more lowerpower class eNBs 208 may have cellular regions 210 that overlap with oneor more of the cells 202. A lower power class eNB 208 may be referred toas a remote radio head (RRH). The lower power class eNB 208 may be afemto cell (e.g., home eNB (HeNB)), pico cell, or micro cell. The macroeNBs 204 are each assigned to a respective cell 202 and are configuredto provide an access point to the EPC 110 for all the UEs 206 in thecells 202. There is no centralized controller in this example of anaccess network 200, but a centralized controller may be used inalternative configurations. The eNBs 204 are responsible for all radiorelated functions including radio bearer control, admission control,mobility control, scheduling, security, and connectivity to the servinggateway 116.

The modulation and multiple access scheme employed by the access network200 may vary depending on the particular telecommunications standardbeing deployed. In LTE applications, OFDM is used on the DL and SC-FDMAis used on the UL to support both frequency division duplexing (FDD) andtime division duplexing (TDD). As those skilled in the art will readilyappreciate from the detailed description to follow, the various conceptspresented herein are well suited for LTE applications. However, theseconcepts may be readily extended to other telecommunication standardsemploying other modulation and multiple access techniques. By way ofexample, these concepts may be extended to Evolution-Data Optimized(EV-DO) or Ultra Mobile Broadband (UMB). EV-DO and UMB are air interfacestandards promulgated by the 3rd Generation Partnership Project 2(3GPP2) as part of the CDMA2000 family of standards and employs CDMA toprovide broadband Internet access to mobile stations. These concepts mayalso be extended to Universal Terrestrial Radio Access (UTRA) employingWideband-CDMA (W-CDMA) and other variants of CDMA, such as TD-SCDMA;Global System for Mobile Communications (GSM) employing TDMA; andEvolved UTRA (E-UTRA), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802.20, and Flash-OFDM employing OFDMA. UTRA, E-UTRA, UMTS, LTE and GSMare described in documents from the 3GPP organization. CDMA2000 and UMBare described in documents from the 3GPP2 organization. The actualwireless communication standard and the multiple access technologyemployed will depend on the specific application and the overall designconstraints imposed on the system.

The eNBs 204 may have multiple antennas supporting MIMO technology. Theuse of MIMO technology enables the eNBs 204 to exploit the spatialdomain to support spatial multiplexing, beamforming, and transmitdiversity. Spatial multiplexing may be used to transmit differentstreams of data simultaneously on the same frequency. The data steamsmay be transmitted to a single UE 206 to increase the data rate or tomultiple UEs 206 to increase the overall system capacity. This isachieved by spatially precoding each data stream (i.e., applying ascaling of an amplitude and a phase) and then transmitting eachspatially precoded stream through multiple transmit antennas on the DL.The spatially precoded data streams arrive at the UE(s) 206 withdifferent spatial signatures, which enables each of the UE(s) 206 torecover the one or more data streams destined for that UE 206. On theUL, each UE 206 transmits a spatially precoded data stream, whichenables the eNB 204 to identify the source of each spatially precodeddata stream.

Spatial multiplexing is generally used when channel conditions are good.When channel conditions are less favorable, beamforming may be used tofocus the transmission energy in one or more directions. This may beachieved by spatially precoding the data for transmission throughmultiple antennas. To achieve good coverage at the edges of the cell, asingle stream beamforming transmission may be used in combination withtransmit diversity.

In the detailed description that follows, various aspects of an accessnetwork will be described with reference to a MIMO system supportingOFDM on the DL. OFDM is a spread-spectrum technique that modulates dataover a number of subcarriers within an OFDM symbol. The subcarriers arespaced apart at precise frequencies. The spacing provides“orthogonality” that enables a receiver to recover the data from thesubcarriers. In the time domain, a guard interval (e.g., cyclic prefix)may be added to each OFDM symbol to combat inter-OFDM-symbolinterference. The UL may use SC-FDMA in the form of a DFT-spread OFDMsignal to compensate for high peak-to-average power ratio (PAPR).

FIG. 3 is a diagram 300 illustrating an example of a DL frame structurein LTE. A frame (10 ms) may be divided into 10 equally sized sub-frames.Each sub-frame may include two consecutive time slots. A resource gridmay be used to represent two time slots, each time slot including aresource block. The resource grid is divided into multiple resourceelements. In LTE, a resource block contains 12 consecutive subcarriersin the frequency domain and, for a normal cyclic prefix in each OFDMsymbol, 7 consecutive OFDM symbols in the time domain, or 84 resourceelements. For an extended cyclic prefix, a resource block contains 6consecutive OFDM symbols in the time domain and has 72 resourceelements. Some of the resource elements, as indicated as R 302, 304,include DL reference signals (DL-RS). The DL-RS include Cell-specific RS(CRS) (also sometimes called common RS) 302 and UE-specific RS (UE-RS)(also known as demodulation reference signals (DM-RS)) 304. UE-RS 304are transmitted only on the resource blocks upon which the correspondingphysical DL shared channel (PDSCH) is mapped. The number of bits carriedby each resource element depends on the modulation scheme. Thus, themore resource blocks that a UE receives and the higher the modulationscheme, the higher the data rate for the UE.

FIG. 4 is a diagram 400 illustrating an example of an UL frame structurein LTE. The available resource blocks for the UL may be partitioned intoa data section and a control section. The control section may be formedat the two edges of the system bandwidth and may have a configurablesize. The resource blocks in the control section may be assigned to UEsfor transmission of control information. The data section may includeall resource blocks not included in the control section. The UL framestructure results in the data section including contiguous subcarriers,which may allow a single UE to be assigned all of the contiguoussubcarriers in the data section.

A UE may be assigned resource blocks 410 a, 410 b in the control sectionto transmit control information to an eNB. The UE may also be assignedresource blocks 420 a, 420 b in the data section to transmit data to theeNB. The UE may transmit control information in a physical UL controlchannel (PUCCH) on the assigned resource blocks in the control section.The UE may transmit only data or both data and control information in aphysical UL shared channel (PUSCH) on the assigned resource blocks inthe data section. A UL transmission may span both slots of a subframeand may hop across frequency.

A set of resource blocks may be used to perform initial system accessand achieve UL synchronization in a physical random access channel(PRACH) 430. The PRACH 430 carries a random sequence and cannot carryany UL data/signaling. Each random access preamble occupies a bandwidthcorresponding to six consecutive resource blocks. The starting frequencyis specified by the network. That is, the transmission of the randomaccess preamble is restricted to certain time and frequency resources.There is no frequency hopping for the PRACH. The PRACH attempt iscarried in a single subframe (1 ms) or in a sequence of few contiguoussubframes and a UE can make only a single PRACH attempt per frame (10ms).

FIG. 5 is a diagram 500 illustrating an example of a radio protocolarchitecture for the user and control planes in LTE. The radio protocolarchitecture for the UE and the eNB is shown with three layers: Layer 1,Layer 2, and Layer 3. Layer 1 (L1 layer) is the lowest layer andimplements various physical layer signal processing functions. The L1layer will be referred to herein as the physical layer 506. Layer 2 (L2layer) 508 is above the physical layer 506 and is responsible for thelink between the UE and eNB over the physical layer 506.

In the user plane, the L2 layer 508 includes a media access control(MAC) sublayer 510, a radio link control (RLC) sublayer 512, and apacket data convergence protocol (PDCP) 514 sublayer, which areterminated at the eNB on the network side. Although not shown, the UEmay have several upper layers above the L2 layer 508 including a networklayer (e.g., IP layer) that is terminated at the PDN gateway 118 on thenetwork side, and an application layer that is terminated at the otherend of the connection (e.g., far end UE, server, etc.).

The PDCP sublayer 514 provides multiplexing between different radiobearers and logical channels. The PDCP sublayer 514 also provides headercompression for upper layer data packets to reduce radio transmissionoverhead, security by ciphering the data packets, and handover supportfor UEs between eNBs. The RLC sublayer 512 provides segmentation andreassembly of upper layer data packets, retransmission of lost datapackets, and reordering of data packets to compensate for out-of-orderreception due to hybrid automatic repeat request (HARQ). The MACsublayer 510 provides multiplexing between logical and transportchannels. The MAC sublayer 510 is also responsible for allocating thevarious radio resources (e.g., resource blocks) in one cell among theUEs. The MAC sublayer 510 is also responsible for HARQ operations.

In the control plane, the radio protocol architecture for the UE and eNBis substantially the same for the physical layer 506 and the L2 layer508 with the exception that there is no header compression function forthe control plane. The control plane also includes a radio resourcecontrol (RRC) sublayer 516 in Layer 3 (L3 layer). The RRC sublayer 516is responsible for obtaining radio resources (i.e., radio bearers) andfor configuring the lower layers using RRC signaling between the eNB andthe UE.

FIG. 6 is a block diagram of an eNB 610 in communication with a UE 650in an access network. In the DL, upper layer packets from the corenetwork are provided to a controller/processor 675. Thecontroller/processor 675 implements the functionality of the L2 layer.In the DL, the controller/processor 675 provides header compression,ciphering, packet segmentation and reordering, multiplexing betweenlogical and transport channels, and radio resource allocations to the UE650 based on various priority metrics. The controller/processor 675 isalso responsible for HARQ operations, retransmission of lost packets,and signaling to the UE 650.

The transmit (TX) processor 616 implements various signal processingfunctions for the L1 layer (i.e., physical layer). The signal processingfunctions includes coding and interleaving to facilitate forward errorcorrection (FEC) at the UE 650 and mapping to signal constellationsbased on various modulation schemes (e.g., binary phase-shift keying(BPSK), quadrature phase-shift keying (QPSK), M-phase-shift keying(M-PSK), M-quadrature amplitude modulation (M-QAM)). The coded andmodulated symbols are then split into parallel streams. Each stream isthen mapped to an OFDM subcarrier, multiplexed with a reference signal(e.g., pilot) in the time and/or frequency domain, and then combinedtogether using an Inverse Fast Fourier Transform (IFFT) to produce aphysical channel carrying a time domain OFDM symbol stream. The OFDMstream is spatially precoded to produce multiple spatial streams.Channel estimates from a channel estimator 674 may be used to determinethe coding and modulation scheme, as well as for spatial processing. Thechannel estimate may be derived from a reference signal and/or channelcondition feedback transmitted by the UE 650. Each spatial stream isthen provided to a different antenna 620 via a separate transmitter618TX. Each transmitter 618TX modulates an RF carrier with a respectivespatial stream for transmission.

At the UE 650, each receiver 654RX receives a signal through itsrespective antenna 652. Each receiver 654RX recovers informationmodulated onto an RF carrier and provides the information to the receive(RX) processor 656. The RX processor 656 implements various signalprocessing functions of the L1 layer. The RX processor 656 performsspatial processing on the information to recover any spatial streamsdestined for the UE 650. If multiple spatial streams are destined forthe UE 650, they may be combined by the RX processor 656 into a singleOFDM symbol stream. The RX processor 656 then converts the OFDM symbolstream from the time-domain to the frequency domain using a Fast FourierTransform (FFT). The frequency domain signal comprises a separate OFDMsymbol stream for each subcarrier of the OFDM signal. The symbols oneach subcarrier, and the reference signal, is recovered and demodulatedby determining the most likely signal constellation points transmittedby the eNB 610. These soft decisions may be based on channel estimatescomputed by the channel estimator 658. The soft decisions are thendecoded and deinterleaved to recover the data and control signals thatwere originally transmitted by the eNB 610 on the physical channel. Thedata and control signals are then provided to the controller/processor659.

The controller/processor 659 implements the L2 layer. Thecontroller/processor can be associated with a memory 660 that storesprogram codes and data. The memory 660 may be referred to as acomputer-readable medium. In the UL, the control/processor 659 providesdemultiplexing between transport and logical channels, packetreassembly, deciphering, header decompression, control signal processingto recover upper layer packets from the core network. The upper layerpackets are then provided to a data sink 662, which represents all theprotocol layers above the L2 layer. Various control signals may also beprovided to the data sink 662 for L3 processing. Thecontroller/processor 659 is also responsible for error detection usingan acknowledgement (ACK) and/or negative acknowledgement (NACK) protocolto support HARQ operations.

In the UL, a data source 667 is used to provide upper layer packets tothe controller/processor 659. The data source 667 represents allprotocol layers above the L2 layer. Similar to the functionalitydescribed in connection with the DL transmission by the eNB 610, thecontroller/processor 659 implements the L2 layer for the user plane andthe control plane by providing header compression, ciphering, packetsegmentation and reordering, and multiplexing between logical andtransport channels based on radio resource allocations by the eNB 610.The controller/processor 659 is also responsible for HARQ operations,retransmission of lost packets, and signaling to the eNB 610.

Channel estimates derived by a channel estimator 658 from a referencesignal or feedback transmitted by the eNB 610 may be used by the TXprocessor 668 to select the appropriate coding and modulation schemes,and to facilitate spatial processing. The spatial streams generated bythe TX processor 668 are provided to different antenna 652 via separatetransmitters 654TX. Each transmitter 654TX modulates an RF carrier witha respective spatial stream for transmission.

The UL transmission is processed at the eNB 610 in a manner similar tothat described in connection with the receiver function at the UE 650.Each receiver 618RX receives a signal through its respective antenna620. Each receiver 618RX recovers information modulated onto an RFcarrier and provides the information to a RX processor 670. The RXprocessor 670 may implement the L1 layer.

The controller/processor 675 implements the L2 layer. Thecontroller/processor 675 can be associated with a memory 676 that storesprogram codes and data. The memory 676 may be referred to as acomputer-readable medium. In the UL, the control/processor 675 providesdemultiplexing between transport and logical channels, packetreassembly, deciphering, header decompression, control signal processingto recover upper layer packets from the UE 650. Upper layer packets fromthe controller/processor 675 may be provided to the core network. Thecontroller/processor 675 is also responsible for error detection usingan ACK and/or NACK protocol to support HARQ operations.

With respect to unicast and multicast/broadcast services, a UE mayperform time tracking (also referred to as timing tracking) for updatinga downlink timing for receiving signals associated with those services.Time tracking is an important factor in receiver performance as itallows for a correct starting point of a FFT window such thatinter-symbol interference is minimized. A timing offset determined bytime tracking can be further used for channel estimation in a currentsubframe and is used to update the downlink timing (i.e., starting pointof the FFT window) for a next subframe.

Traditionally, time tracking in current communication systems (e.g., LTERel-8/9/10) may be based on common reference signals (CRS). CRS iswide-band and may be present in all subframes. Reliable time tracking isthus possible and the time tracking may utilize (e.g., average) two ormore subframes for improved performance. In other communication systems(e.g., LTE Rel-11 and beyond), CRS time tracking may either beimpossible or inappropriate. In some subframes or carriers, CRS may notbe present. For example, a communication system (e.g., LTE Rel-11) maydefine additional carrier types that are backward compatible.Accordingly, CRS may not always be present in these carriers in allsubframes. In the additional carrier types, CRS may be present in onlysome subframes. Furthermore, in some scenarios (e.g., coordinatedmultipoint (CoMP) Tx/Rx), use of CRS may be inappropriate for timetracking. In CoMP, control and data may arrive from different cells.Thus, a UE may assume a wrong cell for CRS-based time tracking.

Non-CRS-based time tracking may be needed in certain communicationsystems (e.g., LTE Rel-11 and beyond). As such, time tracking may beperformed based on UE-RS, CSI-RS, and/or other reference signals.However, unlike CRS-based time tracking, UEs cannot always rely on UE-RSand/or CSI-RS for time tracking, as UE-RS/CSI-RS bandwidth/density maybe limited. For example, UE-RS may be present for a UE only when the UEis scheduled. That is, UE-RS is available for a UE only in a scheduledPDSCH bandwidth, which may range from one resource block (RB) (alsoreferred to as a physical RB (PRB)) to N_(RB) ^(DL) RBs, where N_(RB)^(DL) is a downlink system bandwidth in units of RBs. Accordingly, aUE-RS antenna port may not map to the same physical antenna port(s) overdifferent subframes. Furthermore, CSI-RS may only be present in a subsetof subframes and may have a low density, as only one resource element(RE) per RB per CSI-RS port exists. As a result, time tracking cannotreadily rely on multiple subframes for averaging to improve receiverperformance. In addition, UE-RS-based time tracking performance can beheavily compromised if an assigned PDSCH bandwidth is very small (e.g.,one or a few RBs). Further, CSI-RS time tracking performance can becompromised as well due to low density.

In a communication system (e.g., LTE Rel-11), an enhanced PDCCH (ePDCCH)may be provided. Unlike a legacy PDCCH, which occupies the first severalcontrol symbols in a subframe, ePDCCH occupies a data region, similar toPDSCH. Unlike PDSCH, whose bandwidth is often large, one ePDCCH may onlyconsume one RB or a very limited number of RBs. UE-RS-based ePDCCH maybe supported in the communication system. A UE configured to useePDCCH/PDCCH may not reliably receive CRS (e.g., due to overwhelminginterference from neighboring cells in heterogeneous networks) or CRSmay not be available (e.g., CRS is not present). Time tracking wouldthus not only impact PDSCH performance, but also impact PDCCH/ePDCCHperformance, especially when the corresponding bandwidth is limited.Accordingly, methods are needed to improve the time tracking performancefor UE-RS and/or CSI-RS based time tracking, especially when theassigned bandwidth and/or the RS density is low (in frequency and/ortime).

FIG. 7 is a diagram 700 illustrating the location of UE-RS withinvarious types of subframes and for describing exemplary methods forUE-RS-based time tracking. In FIG. 7, diagram 710 illustrates thelocation of UE-RS within a normal subframe. Diagram 720 illustrates thelocation of UE-RS within a downlink pilot time slot (DwPTS) subframewith 11, 12 symbols. Diagram 730 illustrates the location of UE-RSwithin a DwPTS subframe with 9, 10 symbols. A UE may be able to utilizeup to 24 REs per RB for UE-RS. For rank 1 and rank 2 transmissions, onlyUE-RS REs 702 (darker shaded REs in FIG. 7) are present, resulting in 12REs per RB for UE-RS. For transmissions greater than rank 2, both theUE-RS REs 702 and UE-RS REs 704 (lighter shaded REs in FIG. 7) arepresent, resulting in 24 REs per RB for UE-RS. For a limited number ofRBs (PDSCH and/or ePDCCH), time tracking performance may be heavilycompromised, especially when only 12 UE-RS REs/RB are available (i.e.,when a UE is configured to receive rank 1 or rank 2 transmissions).

To address the UE-RS-based time tracking issue, in a first exemplarymethod, an eNB may explicitly restrict small PDSCH/PDCCH assignments.For example, an eNB may not allow 1 RB PDSCH assignments. Alternatively,an eNB may restrict small PDSCH/PDCCH assignments based on modulationorder. For example, an eNB may allow a 1 RB+QPSK combination, but notcombinations of 1 RB with 16/64-QAM. For a higher modulation order,higher accuracy time tracking may be required. Therefore, the eNB mayrestrict small PDSCH/PDCCH assignments only for higher modulationorders.

According to the first exemplary method, an eNB may restrict a number ofRBs that can be allocated to a target UE in a downlink assignment to begreater than or equal to two (three, four, etc.), generally or based onthe modulation order. The term “target UE” may refer to a UE within theeNB's cell that is the focus of the resource allocation. For example,the eNB may restrict the number of RBs that can be allocated to thetarget UE in the downlink assignment to be two or three. Accordingly,the eNB may transmit a downlink transmission (data/control)corresponding to the downlink assignment to the target UE. Moreover, theeNB may determine a modulation order (e.g., QPSK, 16-QAM, 64-QAM) forthe downlink transmission and determine whether to restrict the numberof RBs based on the determined modulation order. For example, the eNBmay restrict the number of RBs that can be allocated to the target UE inthe downlink assignment to be greater than or equal to two only when themodulation order is greater than a threshold (e.g., the threshold may beQPSK).

In a second exemplary method, a target UE may utilize a precodingresource block group (PRG) feature for UE-RS-based time tracking In acommunication system (e.g., LTE Rel-10), PRGs may be supported for atarget UE configured with transmission mode 9 and precoding matrixindicator (PMI)/rank indicator (RI) channel feedback. In the secondexemplary method, the target UE may assume that a precoding granularityis two or more RBs in the frequency domain (instead of one RB as in atypical case). Accordingly, the target UE may assume that the sameprecoder applies on all scheduled RBs within a PRG. Each PRG includesconsecutive RBs with the same precoding. A PRG size is downlink systembandwidth dependent as shown in the following table:

System Bandwidth (RBs) PRG Size (RBs) ≦10 1 11-26 2 27-63 3  64-110 2

When the target UE receives a plurality of RBs containing UE-RS in aPRG, the target UE may decode the UE-RS in the plurality of resourceblocks based on an assumed same precoding for the transmission of theRBs in the PRG. The target UE may then be able to perform time trackingbased on a coherent combining of the decoded UE-RS. Alternatively, ifthe target UE decodes UE-RS in the plurality of RBs based on a differentprecoding, the target UE would have to independently perform timetracking for each precoding subgroup, and combine the results afterward(e.g., via averaging), which has poorer performance than coherentcombining. As such, in coherent combining, the time tracking algorithmis performed once with all the decoded UE-RS, whereas without coherentcombining, time tracking is performed multiple times and combinedthrough averaging. Accordingly, in the second exemplary method, thetarget UE may exploit the same precoding within the PRG for improvedperformance of UE-RS-based time tracking.

In a third exemplary method, an RB may be assumed to include 24 UE-RSREs (12 UE-RS REs 702 (darker shaded REs in FIG. 7) and 12 UE-RS REs 704(lighter shaded REs in FIG. 7)) for improved time tracking. Theadditional second set of 12 UE-RS (e.g., the 12 UE-RS REs 704) helpsimprove time tracking (and PDSCH/ePDCCH decoding). The additional secondset of 12 UE-RS may be associated with assignments of only a smallnumber of RBs. For example, when a downlink assignment includes one RBPDSCH and/or PDCCH, the RB may be assumed to include 24 UE-RS REs.Otherwise, when a downlink assignment includes more than one RB PDSCHand/or PDCCH, the RB may be assumed to include 12 UE-RS REs or 24 UE-RSREs depending on the configuration or assignments. For example, rank 1or rank 2 transmissions may include 12 UE-RS REs and transmissionsgreater than rank 2 transmissions may include 24 UE-RS REs. A target UEmay perform blind detection to determine the existence of the additional12 UE-RS REs without signaling from the eNB.

The second set of 12 UE-RS REs may not be meant for other UEs.Therefore, the eNB may use the same precoding to transmit the second setof 12 UE-RS REs as used to transmit the first set of 12 UE-RS REs.Alternatively, the second set of 12 UE-RS REs may be meant for otherUEs, and therefore may have a different precoding. For example, thesecond set of UE-RS REs may be used for multiuser MIMO (MU-MIMO)operation for PDSCH or for MU-MIMO operation for ePDCCH. In anotherexample, two or more ePDCCHs may share the same RB in a frequencydivision multiplexing (FDM) manner and use different antenna ports.

Accordingly, depending on whether the second set of UE-RS REs is meantfor other UEs or not, the second set UE-RS REs may be transmitted with adifferent precoding as the precoder used for the first set UE-RS REs.For example, if an eNB transmits the second set of UE-RS REs with adifferent precoding, the second set of UE-RS REs is used by at least oneother UE. In another example, if an eNB transmits the second set ofUE-RS REs with the same precoding as the first set of UE-RS REs (inorder to achieve power randomization), the second set of UE-RS REs maybe used by at least one other UE or by no other UEs. When the second setof UE-RS REs are used by no other UEs, and the target UE is notconfigured to receive the second set of UE-RS REs (e.g., in rank 1 or 2transmissions), the eNB specifically transmits the second set of UE-RSREs to the target UE to enable improved time tracking performance.

Both the first and second sets of UE-RS REs having the same precoding issimilar to a PRG model, the difference being that the same precoding isused across the two sets of UE-RS REs within an RB instead of across RBsas in the PRG case. If both antenna port 7 and antenna port 8 arepresent in the first set of UE-RS REs, the duplication of precoding canbe applied to one of antenna port 7 or antenna port 8 or both of antennaports 7 and 8 depending on whether one of antenna port 7 or antenna port8 or both of antenna ports 7 and 8 are relied on for UE-RS-based timetracking. For PDSCH, because MU-MIMO is only supported with antenna port7 and antenna port 8 (which are mapped to the first set of 12 UE-RSREs), an eNB may use the same precoding for the first and second sets ofUE-RS REs when the target UE is configured to receive the downlinktransmission with antenna port 7 and/or antenna port 8. That is, whenthe target UE is configured to receive the downlink transmission withantenna port 7 and/or antenna port 8, the second set of UE-RS REs arenot utilized by other UEs. Therefore, the eNB may transmit the secondset of UE-RS REs to a particular UE with the same precoding even thoughthe particular UE is configured to receive only the first set of UE-RSREs (e.g., when configured to receive rank 1 or 2 transmissions) toenable better time tracking for the particular UE.

For e-PDCCH, within one RB, antenna port 7 and/or antenna port 8 may bepresent. Alternatively, within one RB, at least one of antenna ports 7,8, 9, or 10 may be present. If only antenna ports 7 and/or 8 are presentfor ePDCCH within one RB, the eNB may use the same precoding for boththe first and second sets of UE-RS REs, as there is no MU-MIMO for thesecond set of UE-RS REs. If antenna ports 7 and/or 8 and antenna ports 9and/or 10 are present for ePDCCH within one RB, the second set of UE-RSREs may be used by other UEs. Accordingly, the eNB may inform a targetUE whether the second set of UE-RS REs is meant for other UEs. If thetarget UE receives such information, the target UE will know that thesecond set of UE-RS REs have a different precoding than the first set ofUE-RS REs. Otherwise, if the target UE does not receive suchinformation, the target UE will assume the second set of UE-RS REs havethe same precoding as the first set of UE-RS REs. The eNB may inform thetarget UE whether the second set of UE-RS REs is used for other UEs viaeither one-bit signaling or through hard-coded information (e.g., noMU-MIMO for one RB operation).

Additionally, if a UE receives two or more ePDCCH transmissions in thesame PRB pair or PRB pairs of the same PRG, the UE may assume that thesame precoding is applied to different antenna ports associated with thetwo or more ePDCCH transmissions. The two or more ePDCCH transmissionsmay be localized transmissions, such that the resources occupied byePDCCH are within the given PRB pair or PRB pairs of the same PRG. Thetwo or more ePDCCH transmissions for the same UE may be a broadcastePDCCH (e.g., system information broadcast), a group cast ePDCCH (e.g.,group power control), a unicast ePDCCH scheduling downlink channels, aunicast ePDCCH scheduling uplink channels, or a combination thereof.

The two or more ePDCCH transmissions may be associated with antennaports 7, 8, 9, and/or 10. For example, the UE may receive an ePDCCH fordownlink scheduling associated with antenna port 7 and an ePDCCH foruplink scheduling with antenna port 8. The UE may assume that the sameprecoding is applied for antenna port 7 and antenna port 8 for the twoePDCCH transmissions. The UE may further assume that the same precodingis also applied for antenna port 9 and 10, if the second set of UE-RSREs are available for ePDCCH decoding for the UE.

In another example, the UE may receive an ePDCCH for downlink schedulingassociated with antenna port 7 and an ePDCCH for uplink scheduling withantenna port 9. The UE may assume that the same precoding is applied forantenna port 7 and antenna port 9 for the two ePDCCH transmissions. Inorder to determine whether or not two or more ePDCCH transmissions inthe same PRB pair or PRB pairs of the same PRG are transmitted to theUE, the UE may perform ePDCCH decoding in a parallel or a serial manner.In parallel decoding, the UE may assume that two or more ePDCCHtransmissions are present such that the same precoding is applied to twoor more antenna ports and perform decoding for the two or more ePDCCHtransmissions simultaneously. In serial decoding, the UE may performePDCCH decoding with one antenna port first, and after at least onesuccessful ePDCCH decoding, the UE may further perform additional ePDCCHdecoding within the same PRB pair or PRB pairs of the same PRG assumingthe same precoding of the corresponding antenna ports. In order tofacilitate the ePDCCH decoding, the eNB may ensure that the two or moreePDCCH transmissions for the same UE are located in the same PRB pair orPRB pairs of the same PRG.

The possible combinations of antenna ports associated with the two ormore ePDCCH transmissions for the same UE can be restricted. Forexample, a UE may assume that only one combination of antenna ports,e.g., {7, 9}, can be used for two ePDCCH transmissions for the same UEin the same PRB pair or PRB pairs of the same PRG.

In one configuration, if four ports in an RB are necessary, an eNB maybe configured to use antenna ports 7, 8, 11, and 13 (which are mapped tothe first set of 12 UE-RS REs), and therefore a target UE may assume thesame precoding for both the first and second sets of UE-RS REs.Alternatively, an eNB may be configured to use antenna ports 9, 10, 12,and 14 (which are mapped to the second set of 12 UE-RS REs), andtherefore a target UE may assume the same precoding for both the firstand second sets of UE-RS REs.

The three design alternatives are not necessarily exclusive to eachother. That is, additional exemplary methods may combine at least two ofthe first, second, and third exemplary methods. For example, acombination of the second exemplary method and the third exemplarymethod for a rank 1 or rank 2 transmission of PDSCH with 2 RBs in thesame PRG may be used, where 24 UE-RS REs are present in each of the 2RBs, and each set of 12 REs has the same precoder.

FIG. 8 illustrates diagrams 800, 802, and 804 of reference signalconfigurations within a set of resource blocks and for describingexemplary methods for CSI-RS-based time tracking. The set of resourceblocks may include common or cell-specific reference signals (CRS) forports 1, 2, 3, and 4, demodulation reference signals (DM-RS), andchannel state information reference signals (CSI-RS). Diagram 800 showsa configuration for two CSI-RS, diagram 802 shows a configuration forfour CSI-RS, and diagram 804 shows a configuration for eight CSI-RS. Aphysical downlink control channel (PDCCH) and the PDSCH are also shown.For CSI-RS, there is one RE per CSI-RS port, and up to eight CSI-RSports per set of CSI-RS resources. Currently, CSI-RS has a smallestperiodicity of 5 ms (present once every 5 ms at most). In acommunication system (e.g., LTE Rel-11), a target UE may be configuredwith two or more sets of CSI-RS resources. For example, if the target UEis configured with two sets of CSI-RS resources and each set includeseight CSI-RS ports, then the target UE may be configured with 16 CSI-RSports.

In a fourth exemplary method, a target UE may assume that it isconfigured with eight CSI-RS ports, even if there is a smaller number ofrequired CSI-RS ports (e.g., 4). If CSI-RS is precoded, the sameprecoding can be applied to the additionally assumed CSI-RS ports.Additionally, an eNB can broadcast the proximity of the two or moreCSI-RS resource sets. Typically, the two or more configured sets ofCSI-RS resources for a target UE may belong to different cells (e.g., inCoMP), which can be physically non-collocated. However, if the two ormore cells have roughly the same distance to the target UE, such thatthe cells have approximately the same downlink timing at target UEreception (including propagation delay, repeater delay, etc.), thetarget UE can safely combine the two or more CSI-RS sets for improvedtime tracking. Otherwise, the target UE may not combine the two or moreCSI-RS sets for time tracking, as they may have different downlinktimings at the target UE reception. By informing the target UE of theproximity of the two or more CSI-RS sets, the target UE can takeappropriate action for time tracking.

The assumed additional CSI-RS may be narrowband. Therefore, a target UEmay assume that the additional CSI-RS exist in only a subset of thesubframes that carry CSI-RS. As such, there may be one set of widebandCSI-RS for CSI feedback and a different set of narrowband CSI-RS (whichis only localized to the assigned PDSCH bandwidth) for time tracking.For backward compatibility with legacy UEs, the eNB may need tobroadcast the second set of CSI-RS as muted REs for the legacy UEs. Theexemplary methods may also be applied to channel estimation forPDSCH/ePDCCH or other functions. Combinations of UE-RS and CSI-RS basedon approaches are also possible.

FIG. 9 is a diagram 900 for illustrating the exemplary methods. In afirst configuration, an eNB 902 restricts a number of RBs that can beallocated to a UE 904 in a downlink assignment 909 to be greater than orequal to N, where N>1 (e.g., N=2). The eNB 902 transmits the downlinktransmission 910 corresponding to the downlink assignment 909 to the UE904. The eNB 902 may determine a modulation order for the downlinktransmission 910 and determine whether to restrict the number of RBsthat can be allocated to the UE 904 based on the determined modulationorder. The eNB 902 may restrict the number of RBs based on thedetermined modulation order by restricting the number of RBs that can beallocated to the UE 904 in the downlink assignment 909 to be greaterthan or equal to two only when the modulation order is greater than athreshold (e.g., 1 RB+QPSK is allowed, but 1 RB+16-QAM is not allowed).Based on the UE-RS and/or CSI-RS in the plurality of RBs within thedownlink transmission 910, the UE 904 performs time tracking 912.

In a second configuration, the UE 904 receives a plurality of RBs in atransmission 910 from the eNB 902. The plurality of RBs includes a PRG.The UE 904 decodes UE-RS based on an assumed same precoding fortransmission of the RBs in the PRG. The UE 904 performs time tracking912 based on the decoded UE-RS in the PRG. The UE 904 may receive aconfiguration from the eNB 902 to receive the transmission 910 using atransmission mode supporting CoMP transmission (e.g., transmission mode9).

In a third configuration, the UE 904 receives at least one RB in atransmission 910. Each of the at least one RB includes a first set ofUE-RS (e.g., the UE-RS 702 including 12 UE-RS REs). The UE 904determines whether an RB of the at least one RB includes a second set ofUE-RS (e.g., the UE-RS 704 including 12 UE-RS REs). The UE 904 performstime tracking 912 based on the first set of UE-RS and based on thesecond set of UE-RS when the RB is determined to include the second setof UE-RS. The UE 904 may determine whether the RB of the at least one RBincludes the second set of UE-RS only when the at least one RB includesonly the RB. That is, the UE 904 may determine (e.g., through blinddetection or explicit signaling) whether the second set of UE-RS isincluded in the downlink transmission only when less than a thresholdnumber (e.g., 2 RBs) of RBs are received. The transmission 910 may be arank one transmission or a rank two transmission. The first set of UE-RSmay be meant for the UE 904, and the second set of UE-RS may be meantfor another UE (e.g., the UE 908) or no other UEs. The first set ofUE-RS and the second set of UE-RS may have a different precoding and,therefore, the second set of UE-RS may be meant for another UE, such asthe UE 908. Of course, the eNB may transmit the second set of UE-RS witha different precoding than used for the first set of UE-RS even thoughthe second set of UE-RS is for no other UEs. The first set of UE-RS andthe second set of UE-RS may have the same precoding. The UE 904 maydetermine whether an RB of the at least one RB includes the second setof UE-RS by performing blind detection. The UE 904 may determine whetheran RB of the at least one RB includes the second set of UE-RS byreceiving from the eNB 902 information indicating whether the RBincludes the second set of UE-RS.

In a fourth configuration, the eNB 902 configures the UE 904 to receiveone of a rank 1 transmission or a rank 2 transmission. The eNB 902transmits an RB to the UE 904. The RB includes a first set of UE-RS anda second set of UE-RS. One of the first set of UE-RS and the second setof UE-RS is meant for the UE 904. The other one of the first set ofUE-RS and the second set of UE-RS is meant for another UE (e.g., the UE908) or no other UEs. The first set of UE-RS may include 12 UE-RS andthe second set of UE-RS may include 12 UE-RS for a total of 24 UE-RS.The eNB 902 may transmit the RB with a set of RBs, and the eNB 902 maytransmit the second set of UE-RS with at least one UE-RS when a numberof RBs in the set of RBs is less than a threshold number (e.g., 2). TheeNB 902 may use the same precoding for the first set of UE-RS and thesecond set of UE-RS. The eNB 902 may use a different precoding for thefirst set of UE-RS and the second set of UE-RS, and therefore the otherone of the first set of UE-RS and the second set of UE-RS is for anotherUE, such as the UE 908. The eNB 902 may determine to transmit the RBwith four antenna ports, and to select the four antenna ports to includeantenna ports 7, 8, 11, and 13. The antenna ports 7, 8, 11, and 13 mayprovide a mapping of UE-RS to the first set of UE-RS. The first set ofUE-RS may be for the UE 904 and the second set of UE-RS may be for noother UEs, and the eNB 902 may include the second set of UE-RS in the RBto enable the UE 904 to perform time tracking based on both the firstset of UE-RS and the second set of UE-RS. The eNB 902 may determine totransmit the RB with four antenna ports, and to select the four antennaports to include antenna ports 9, 10, 12, and 14. The antenna ports 9,10, 12, and 14 may provide a mapping of UE-RS to the second set ofUE-RS. The second set of UE-RS may be for the UE 904 and the first setof UE-RS may be for no other UEs, and the eNB 902 may include the firstset of UE-RS in the RB to enable the UE 904 to perform time trackingbased on both the first set of UE-RS and the second set of UE-RS.

In a fifth configuration, the UE 904 receives a configuration to receiveat least one RB with a first number of CSI-RS ports (e.g., four CSI-RSports) in each RB of the at least one RB. The UE 904 receives the atleast one RB in a transmission 910. The UE 904 assumes that an RB of theat least one RB includes a second number of CSI-RS ports (e.g., 8 CSI-RSports) greater than the first number of CSI-RS ports. The UE 904performs time tracking 912 based on signals in REs corresponding to theassumed second number of CSI-RS ports. The UE 904 may assume that the RBincludes the second number of CSI-RS ports when the at least one RBincludes less than a threshold number (e.g., 8) of RBs and the firstnumber of CSI-RS ports is less than a threshold number of CSI-RS ports.The second number of CSI-RS ports may include at least a first set ofCSI-RS ports transmitted by the eNB 902 and a second set of CSI-RS portstransmitted by a second eNB 906. The UE 904 may receive proximityinformation from the eNB 902 indicating one of a distance from each ofthe eNB 902 and the eNB 906, a propagation time from each of the eNB 902and the eNB 906, a relationship between the distance from the eNB 902and the eNB 906, or a relationship between the propagation time from theeNB 902 and the eNB 906. The UE 904 may perform time tracking 912 basedon the received proximity information.

In a sixth configuration, the eNB 902 configures the UE 904 to receive afirst number of CSI-RS ports. The eNB 902 transmits to the UE 904 an RBincluding a second number of CSI-RS ports greater than the first numberof CSI-RS ports. The second number of CSI-RS ports enables improved timetracking by the UE 904. The first number of CSI-RS ports may correspondto a first set of CSI-RS ports, additional CSI-RS ports in the secondnumber of CSI-RS ports may correspond to a second set of CSI-RS ports,and the same precoding may be applied to the first set of CSI-RS portsand the second set of CSI-RS ports in the transmitted RB. The eNB 902may transmit proximity information to the UE 904. The proximityinformation may include one of a distance between the UE 904 and the eNB902 and a distance between the UE 904 and a second eNB 906, apropagation time from the eNB 902 to the UE 904 and from the second eNB906 to the UE 904, a relationship between the distance from the eNB 902and the second eNB 906, or a relationship between the propagation timefrom the eNB 902 and the second eNB 906. The first number of CSI-RSports may correspond to a first configuration, additional CSI-RS portsin the second number of CSI-RS ports as compared to the first number ofCSI-RS ports may correspond to a second configuration, and the eNB 902may transmit a second RB including CSI-RS ports corresponding to onlythe first configuration.

In a seventh configuration, the UE 904 receives UE-RS and CSI-RS in atleast one RB, and performs time tracking 912 based on the received UE-RSand CSI-RS.

UE-RS Based Timing Estimation

In coordinated multipoint (CoMP) Tx/Rx, control and data may come fromdifferent cells. Thus, there may be a timing difference between controland data transmissions. If CRS is used to perform time tracking, acontrol channel may be tracked, and may have the correct timing.However, the timing of a data channel may be misaligned. Accordingly,use of CRS may be inappropriate for time tracking in such a scenario.

In an aspect, UE-RS may be used to improve the time tracking. Becausethe UE-RS is bound to data, a UE-RS signal may be used to measure timingfor the data channel. Time tracking based on UE-RS may be applied whentransmissions via antenna ports 7 and 8 have different timing.

In principle, a timing error Δt of a channel in the time domaincorresponds to phase ramping in the frequency domain. A channel h may bedefined by the equation:h(k,m,n)=h ₀(k,m,n)e ^(−jnωΔt) +n ₀(k,m,n),  (1)where h₀ is the original channel, k is a resource block (RB) index, m isa symbol index, n is a tone index, and n₀ is an additive white Gaussiannoise (AWGN) component.

FIG. 10 is a diagram 1000 illustrating positions of UE-RS signalsreceived via antenna ports 7 and 8 in a subframe. An original UE-RSsignal may be defined as x, where x is a multiplexed signal havingantenna port 7 and antenna port 8 transmissions multiplexed on oneresource. The antenna port 7 and antenna port 8 transmissions may havethe same or different phase ramping. For a resource pair (e.g., a pairor resources of adjacent OFDM symbols having the same tone and withinthe same RB), each resource may receive an original UE-RS signal xhaving multiplexed antenna port 7 and antenna port 8 transmissionsdefined by the following equations:x ₁(k,m,n)=h _(—) P7+h _(—) P8=a; and  (2)x ₂(k,m+1,n)=h _(—) P7−h _(—) P8=b,  (3)where x₁ is the original UE-RS signal received in the first resource ofthe resource pair, x₂ is the original UE-RS signal received in thesecond resource of the resource pair, k is a resource block (RB) index,m is a symbol index, n is a tone index, h_P7 is the antenna port 7transmission, and h_P8 is the antenna port 8 transmission.

After the original UE-RS signal x is received, x may be de-multiplexedor de-spread. y may be defined as the received UE-RS signal afterde-spreading. With reference to Equations (2) and (3) above, for theresource pair, the value of y in each resource is determined by thefollowing equations:y ₁(k,m,n)=h _(—) P7=(a+b)/2; and  (4)y ₂(k,m+1,n)=h _(—) P8=(a−b)/2,  (5)where y₁ is the value of the de-spread UE-RS signal in the firstresource of the resource pair, and y₂ is the value of the de-spreadUE-RS signal in the second resource of the resource pair. For example,in FIG. 10, for a pair of resources 1002 and 1008, y₁ is the value ofthe de-spread UE-RS signal in the resource 1002, and y₂ is the value ofthe de-spread UE-RS signal in the resource 1008.

Referring to FIG. 10, after the original UE-RS signal is de-spread, atotal of six y values (i.e., six de-spread UE-RS signal values) in eachRB may be determined. For example, in RB 1, y values may be determinedat resources 1002, 1004, and 1006 in OFDM symbol 5 and at resources1008, 1010, and 1012 in OFDM symbol 6. In RB 2, y values may bedetermined at resources 1014, 1016, and 1018 in OFDM symbol 5 and atresources 1020, 1022, and 1024 in OFDM symbol 6. Accordingly, thesubframe comprising RBs 1 and 2 may have a total of 12 y values.

Furthermore, the de-spread UE-RS signal values may be used for channelestimation of a specific antenna port. For example, referring to FIG.10, the UE-RS signal values at the resources 1002, 1004, and 1006 inOFDM symbol 5 of RB 1, and the UE-RS signal values at the resources1014, 1016, and 1018 in OFDM symbol 5 of RB 2, may be used for channelestimation for antenna port 7. Also, the UE-RS signal values at theresources 1008, 1010, and 1012 in OFDM symbol 6 of RB 1, and the UE-RSsignal values at the resources 1020, 1022, and 1024 in OFDM symbol 6 ofRB 2, may be sued for channel estimation for antenna port 8.

In FIG. 10, it is noted that within an OFDM symbol, each UE-RS signal isseparated from a nearest adjacent UE-RS signal by a distance of fivetones. In an aspect, it is assumed that an original channel h₀ (withoutphase ramping) remains constant over two adjacent UE-RS signal values(75 kHz coherent bandwidth) in an OFDM symbol. Hence, h₀(k, m, n)≈h₀(k,m, n+5), where k is a resource block (RB) index, m is a symbol index,and n is a tone index.

Moreover, in each OFDM symbol, there are three UE-RS signal values thatare assumed to be equal, wherein the three UE-RS signal valuescorrespond to two pairs. For example, in FIG. 10, the UE-RS signalvalues at the resources 1002, 1004, and 1006 in OFDM symbol 5 of RB 1are assumed to be equal, and correspond to two pairs. The first pair mayinclude a first UE-RS signal value at the resource 1002 and a secondUE-RS signal value at the resource 1004. The second pair may include thesecond UE-RS signal value at the resource 1004 and a third UE-RS signalvalue at the resource 1006. A similar pairing configuration applies tothe UE-RS signal values in OFDM symbol 6 of RB 1 and OFDM symbols 5 and6 of RB 2.

If it is assumed that the original channel h₀ in each pair is constant,and if there is no timing error between the first and second UE-RSsignal values of the first pair, then the channel is the same.Furthermore, if there is no timing error between the second and thirdUE-RS signal values of the second pair, then the channel is the same.Accordingly, a connection may be drawn between y values in an OFDMsymbol that is defined by the following equation:y(k,m,n+5)=y(k,m,n)e ^(−jω5Δt)ξ(k,m,n),  (6)where k is a resource block (RB) index, m is a symbol index, n is a toneindex, and is a noise component. Equation (6) may be used to performtiming estimation. For example, from Equation (6), a maximum ratiocombiner (MRC) may be used to estimate a frequency ramping term (e.g.construct a sample) from y.

In a configuration, the UE 904 receives at least one resource blocks ina transmission, each of the at least one resource blocks comprising afirst group of UE-reference signals (UE-RS) associated with a firstantenna port. The UE 904 then determines whether the at least oneresource blocks comprise a second group of UE-RS associated with one ormore other antenna ports, and processes the received at least oneresource blocks based on the first group of UE-RS, and further based onthe second group of UE-RS when the at least one resource blocks isdetermined to comprise the second group of UE-RS. The transmission maybe associated with a control channel transmission, a data channeltransmission, or a combination thereof.

The UE 904 may process the received at least one resource blocks byconstructing a number of samples for performing at least one of channelestimation or timing estimation. The first group of UE-RS and the secondgroup of UE-RS may be multiplexed in a code domain sharing the samefrequency-time resources. Moreover, the first group of UE-RS and thesecond group of UE-RS may be transmitted in orthogonal frequency-timeresources. Also, a same precoding may be applied to the first group ofUE-RS and the second group of UE-RS.

If downlink control information (DCI) signals a rank 2 transmission ortransmission greater than rank 2, then the transmission from the firstantenna port and the one or more antenna ports come from the sameevolved Node B (eNB), and thus have a common phase ramping. Accordingly,the UE 904 determines that the at least one resource blocks comprise thesecond group of UE-RS and constructs samples based on the first group ofUE-RS associated with the first antenna port and samples based on thesecond group of UE-RS associated with the one or more other antennaports for each of the at least one resource blocks. Thereafter, the UE904 processes the received at least one resource blocks by processingthe combined samples based on the first group of UE-RS and based on thesecond group of UE-RS.

If the DCI signals a rank 1 transmission, then the UE 904 determineswhether the transmission is a single-user transmission or a multi-usertransmission. The UE 904 may utilize a single-user/multi-user detectorto distinguish between the single-user transmission and the multi-usertransmission. Otherwise, the UE 904 may determine the transmission to bethe single-user transmission or the multi-user transmission according toa bit received from an eNB indicating either the single-usertransmission or the multi-user transmission.

When the UE 904 determines the transmission to be the single-usertransmission, the UE 904 determines that the at least one resourceblocks does not comprise the second group of UE-RS associated with asecond antenna port. Accordingly, the UE 904 constructs samples based onthe first group of UE-RS associated with the first antenna port for eachof the at least one resource blocks. Thereafter, the UE 904 processesthe received at least one resource blocks by processing the constructedsamples based on the first group of UE-RS associated with the firstantenna port.

When the UE 904 determines the transmission to be the multi-usertransmission, the UE 904 determines that the at least one resourceblocks comprise the second group of UE-RS associated with a secondantenna port. Here, the transmissions received via the first antennaport and the second antenna port may come from different eNBs, andtherefore may have different phase ramping. As such, because thetransmissions cannot be combined, the transmissions associated with thefirst antenna port and the second antenna port should be estimatedseparately. Accordingly, the UE 904 further determines whether the firstgroup of UE-RS and second group of UE-RS are received from a same eNB ordifferent eNBs. The UE 904 may utilize a single-user/multi-user detectorto determine whether the first group of UE-RS and second group of UE-RSare received from the same eNB or different eNBs.

When the first group of UE-RS and second group of UE-RS are receivedfrom the same eNB, the UE 904 constructs samples based on the firstgroup of UE-RS associated with the first antenna port and samples basedon the second group of UE-RS associated with the second antenna port foreach of the at least one resource blocks. Thereafter, the UE 904processes the received at least one resource blocks by processing thecombined samples based on the first group of UE-RS and based on thesecond group of UE-RS.

When the first group of UE-RS and second group of UE-RS are receivedfrom different eNBs, the UE 904 constructs samples based on the firstgroup of UE-RS associated with the first antenna port for each of the atleast one resource blocks. Thereafter, the UE 904 processes the receivedat least one resource blocks by processing the constructed samples basedon the first group of UE-RS associated with the first antenna port.

Alternatively, when the first group of UE-RS and second group of UE-RSare received from different eNBs, the UE 904 constructs samples based onthe second group of UE-RS associated with the second antenna port foreach of the at least one resource blocks. Thereafter, the UE 904processes the received at least one resource blocks by processing theconstructed samples based on the second group of UE-RS associated withthe second antenna port.

FIG. 11 is a flow chart 1100 of a method of wireless communication. Themethod may be performed by an eNB. At step 1102, the eNB may determine amodulation order for a downlink transmission.

At step 1104, the eNB restricts a number of resource blocks that can beallocated to a user equipment (UE) in a downlink assignment to begreater than or equal to N, where N is greater than one. The restrictionmay be based on the determined modulation order. For example, the eNBmay restrict the number of resource blocks that can be allocated to theUE in the downlink assignment to be greater than or equal to two onlywhen the modulation order is greater than a threshold.

At step 1106, the eNB transmits to the UE the downlink transmissioncorresponding to the downlink assignment.

FIG. 12 is a flow chart 1200 of a method of wireless communication. Themethod may be performed by a UE. At step 1202, the UE may receive aconfiguration to receive a transmission using a transmission modesupporting cooperative multipoint (CoMP) transmission.

At step 1204, the UE receives a plurality of resource blocks in atransmission. The plurality of resource blocks may include a precodingresource block group (PRG).

At step 1206, the UE decodes user equipment specific reference signals(UE-RS) based on an assumed same precoding for transmission of theresource blocks in the PRG. Thereafter, at step 1208, the UE performstime tracking based on the decoded UE-RS in the PRG.

FIG. 13 is a flow chart 1300 of a method of wireless communication. Themethod may be performed by a UE. At step 1302, the UE receives at leastone resource block in a transmission. The transmission may be a rank onetransmission or a rank two transmission. Each of the at least oneresource block may include a first set of user equipment specificreference signals (UE-RS).

At step 1304, the UE determines whether a resource block of the at leastone resource block includes a second set of UE-RS. In an aspect, thedetermination of whether the resource block of the at least one resourceblock includes the second set of UE-RS is performed only when the atleast one resource block comprises only the resource block. In anotheraspect, the determination includes performing blind detection todetermine whether the resource block includes the second set of UE-RS.In a further aspect, the determination includes receiving from an eNBinformation indicating whether the resource block includes the secondset of UE-RS.

At step 1306, the UE performs time tracking based on the first set ofUE-RS and based on the second set of UE-RS when the resource block isdetermined to comprise the second set of UE-RS.

In an aspect, the transmission may be meant for the UE, the first set ofUE-RS may be meant for the UE, and the second set of UE-RS may be meantfor another UE or no other UEs. Accordingly, the first set of UE-RS andthe second set of UE-RS may have a different precoding. Alternatively,the first set of UE-RS and the second set of UE-RS may have the sameprecoding.

FIG. 14 is a flow chart 1400 of a method of wireless communication. Themethod may be performed by an eNB. At step 1402, the eNB configures auser equipment (UE) to receive one of a rank one transmission or a ranktwo transmission.

At step 1404, the eNB determines to transmit a resource block with fourantenna ports. At step 1406, the eNB selects the four antenna ports. Theselected four antenna ports may be antenna ports 7, 8, 11, and 13.Alternatively, the selected four antenna ports may be antenna ports 9,10, 12, and 14.

At step 1408, the eNB transmits the resource block to the UE. Theresource block may include a first set of user equipment specificreference signals (UE-RS) and a second set of UE-RS. One of the firstset of UE-RS and the second set of UE-RS may be meant for the UE. Theother one of the first set of UE-RS and the second set of UE-RS may bemeant for another UE or no other UEs.

The first set of UE-RS may include 12 UE-RS and the second set of UE-RSmay include 12 UE-RS for a total of 24 UE-RS. The resource block may betransmitted with a set of resource blocks. Moreover, the second set ofUE-RS may include at least one UE-RS when a number of resource blocks inthe set of resource blocks is less than a threshold number.

In an aspect, the same precoding may be used for the first set of UE-RSand the second set of UE-RS. Alternatively, different precoding may beused for the first set of UE-RS and the second set of UE-RS, wherein theother one of the first set of UE-RS and the second set of UE-RS is meantfor the other UE.

In a further aspect, the antenna ports 7, 8, 11, and 13 provide amapping of UE-RS to the first set of UE-RS, wherein the first set ofUE-RS is meant for the UE and the second set of UE-RS is meant for noother UEs. Accordingly, the eNB may include the second set of UE-RS inthe resource block to enable the UE to perform time tracking based onboth the first set of UE-RS and the second set of UE-RS.

In another aspect, the antenna ports 9, 10, 12, and 14 provide a mappingof UE-RS to the second set of UE-RS, wherein the second set of UE-RS ismeant for the UE and the first set of UE-RS is meant for no other UEs.Accordingly, the eNB may include the first set of UE-RS in the resourceblock to enable the UE to perform time tracking based on both the firstset of UE-RS and the second set of UE-RS.

FIG. 15 is a flow chart 1500 of a method of wireless communication. Themethod may be performed by a UE. At step 1502, the UE receives aconfiguration to receive at least one resource block with a first numberof channel state information reference signal (CSI-RS) ports in eachresource block of the at least one resource block. At step 1504, the UEreceives the at least one resource block in a transmission.

At step 1506, the UE assumes a resource block of the at least oneresource block includes a second number of CSI-RS ports greater than thefirst number of CSI-RS ports. The UE assumes the resource block tocomprise the second number of CSI-RS ports when the at least oneresource block comprises less than a threshold number of resource blocksand the first number of CSI-RS ports is less than a threshold number ofCSI-RS ports.

At step 1508, the UE determines whether the second number of CSI-RSports comprises at least a first set of CSI-RS ports transmitted by afirst eNB and a second set of CSI-RS ports transmitted by a second eNB.Based on a negative outcome, the UE proceeds to step 1512 to performtime tracking based on signals in resource elements corresponding to theassumed second number of CSI-RS ports.

At step 1510, based on a positive outcome at step 1508, the UE receivesproximity information from a serving eNB. The proximity information mayindicate one of a distance from each of the first eNB and the secondeNB, a propagation time from each of the first eNB and the second eNB, arelationship between the distance from the first eNB and the second eNB,or a relationship between the propagation time from the first eNB andthe second eNB. Thereafter, at step 1512, the UE performs the timetracking further based on the received proximity information.

FIG. 16 is a flow chart 1600 of a method of wireless communication. Themethod may be performed by an eNB. At step 1602, the eNB configures auser equipment (UE) to receive a first number of channel stateinformation reference signal (CSI-RS) ports.

At step 1604, the eNB transmits to the UE a resource block comprising asecond number of CSI-RS ports greater than the first number of CSI-RSports. The second number of CSI-RS ports enables improved time trackingby the UE. In an aspect, the first number of CSI-RS ports corresponds toa first set of CSI-RS ports, additional CSI-RS ports in the secondnumber of CSI-RS ports correspond to a second set of CSI-RS ports, andthe same precoding is applied to the first set of CSI-RS ports and thesecond set of CSI-RS ports in the transmitted resource block.

At step 1606, the eNB transmits proximity information to the UE. Theproximity information may include one of a distance between the UE andthe eNB and a distance between the UE and a second eNB, a propagationtime from the eNB to the UE and from the second eNB to the UE, arelationship between the distance from the eNB and the second eNB, or arelationship between the propagation time from the eNB and the secondeNB.

In an aspect, the first number of CSI-RS ports may correspond to a firstconfiguration and additional CSI-RS ports in the second number of CSI-RSports as compared to the first number of CSI-RS ports may correspond toa second configuration. Accordingly, at step 1608, the eNB may transmita second resource block comprising CSI-RS ports corresponding to onlythe first configuration.

FIG. 17 is a flow chart 1700 of a method of wireless communication. Themethod may be performed by a UE. At step 1702, the UE receives userequipment specific reference signals (UE-RS) and channel stateinformation reference signals (CSI-RS) in at least one resource block.At step 1704, the UE performs time tracking based on the received UE-RSand CSI-RS.

FIG. 18 is a flow chart 1800 of a method of wireless communication. Themethod may be performed by a UE. At step 1802, the UE may receive aconfiguration to receive a transmission using a transmission modesupporting cooperative multipoint (CoMP) transmission. At step 1804, theUE receives at least one resource block in the transmission. The atleast one resource block includes a first set of reference signals (RS),specific to the UE.

At step 1806, the UE determines whether a second set of RS, specific tothe UE, is available in the transmission. The determining operation mayinclude the UE performing blind detection to determine the availabilityof the second set of RS. Alternatively, the determining operation mayinclude the UE receiving from an evolved Node B (eNB) informationindicating whether the second set of RS is available or not.

At step 1808, the UE may receive proximity information from the eNB. Theproximity information may include a propagation time difference of thefirst set of RS and the second set of RS. Thereafter, at step 1810, theUE processes the received at least one resource block based on the firstset of RS and further based on the second set of RS if the second set ofRS is determined to be available. The processing may include performingchannel estimation and/or timing estimation.

In an aspect, the first set of RS and the second set of RS areUE-specific RS (UE-RS) for demodulation. The first set of UE-RS and thesecond set of UE-RS may have a different precoding or the sameprecoding. The availability of the second set of UE-RS may be determinedwhen the second set of UE-RS is contained within the at least oneresource block. Alternatively, the availability of the second set ofUE-RS may be determined when a resource block comprising the second setof UE-RS is associated with a same precoding resource block group (PRG)as the at least one resource block. In another aspect, the transmissionmay be for the UE, the first set of UE-RS may be for the UE, and thesecond set of UE-RS may be for another UE or no other UEs.

In a further aspect, the first set of RS and the second set of RS arechannel state information reference signals (CSI-RS). The first set ofCSI-RS and the second set of CSI-RS may be associated with differentresources. Alternatively, the first set of CSI-RS and the second set ofCSI-RS may be associated with a same set of resources but with differentantenna ports. Moreover, the same precoding may be applied to a firstset of CSI-RS ports and a second set of CSI-RS ports in a transmittedresource block.

In yet another aspect, the first set of RS may include UE-specific RS(UE-RS) for demodulation and the second set of RS may include channelstate information reference signals (CSI-RS).

FIG. 19 is a flow chart 1900 of a method of wireless communication. Themethod may be performed by an evolved Node B (eNB). At step 1902, theeNB configures a user equipment (UE) to receive a transmission. This mayinclude the eNB transmitting to the UE a configuration to receive thetransmission using a transmission mode supporting cooperative multipoint(CoMP) transmission.

At step 1904, the eNB transmits to the UE at least one resource block inthe transmission. The at least one resource block includes a first setof reference signals (RS), specific to the UE. At step 1906, the eNBprovides a second set of RS in the transmission.

At step 1908, the eNB may transmit to the UE information indicatingwhether or not the second set of RS is available to the UE. At step1910, the eNB may also transmit to the UE proximity information. Theproximity information includes a propagation time difference of thefirst set of RS and the second set of RS. Accordingly, the UE mayperform channel estimation and/or timing estimation based on the firstset of RS and further based on the second set of RS if the second set ofRS is available to the UE.

In an aspect, the first set of RS and the second set of RS areUE-specific RS (UE-RS) for demodulation. The first set of UE-RS and thesecond set of UE-RS may have a different precoding or the sameprecoding. The second set of UE-RS may be available to the UE when thesecond set of UE-RS is contained within the at least one resource block.Alternatively, the second set of UE-RS may be available to the UE when aresource block comprising the second set of UE-RS is associated with asame precoding resource block group (PRG) as the at least one resourceblock. In another aspect, the transmission may be for the UE, the firstset of UE-RS may be for the UE, and the second set of UE-RS may be foranother UE or no other UEs.

In a further aspect, the first set of RS and the second set of RS arechannel state information reference signals (CSI-RS). The first set ofCSI-RS and the second set of CSI-RS may be associated with differentresources. Alternatively, the first set of CSI-RS and the second set ofCSI-RS may be associated with a same set of resources but with differentantenna ports. Moreover, the same precoding may be applied to a firstset of CSI-RS ports and a second set of CSI-RS ports in a transmittedresource block.

In yet another aspect, the first set of RS includes UE-specific RS(UE-RS) for demodulation and the second set of RS includes channel stateinformation reference signals (CSI-RS).

FIG. 20 is a conceptual data flow diagram 2000 illustrating the dataflow between different modules/means/components in an exemplaryapparatus 2002. The apparatus may be an eNB. The apparatus includes areceiving module 2004, a modulation order determining module 2006, aresource block processing module 2008, a data processing module 2010, arank configuration module 2012, a port processing module 2014, areference signal processing module 2016, a proximity information module2018, and a transmission module 2020.

The modulation order determining module 2006 may determine a modulationorder for a downlink transmission. The resource block processing module2008 restricts a number of resource blocks that can be allocated to auser equipment (UE) 2050 in a downlink assignment to be greater than orequal to N, where N is greater than one. The restriction may be based onthe modulation order determined by the modulation order determiningmodule 2006. For example, the resource block processing module 2008 mayrestrict the number of resource blocks that can be allocated to the UE2050 in the downlink assignment to be greater than or equal to two onlywhen the modulation order is greater than a threshold. The dataprocessing module 2010 may transmit to the UE 2050 the downlinktransmission corresponding to the downlink assignment.

The rank configuration module 2012 configures the UE 2050 to receive oneof a rank one transmission or a rank two transmission. The resourceblock processing module 2008 may determine to transmit a resource blockwith four antenna ports. As such, the port processing module 2014selects the four antenna ports. The selected four antenna ports may beantenna ports 7, 8, 11, and 13. Alternatively, the selected four antennaports may be antenna ports 9, 10, 12, and 14.

The resource block processing module 2008 transmits the resource blockto the UE 2050 via the transmission module 2020. The resource block mayinclude a first set of user equipment specific reference signals (UE-RS)and a second set of UE-RS generated by the reference signal processingmodule 2016. One of the first set of UE-RS and the second set of UE-RSmay be meant for the UE 2050. The other one of the first set of UE-RSand the second set of UE-RS may be meant for another UE or no other UEs.

The first set of UE-RS may include 12 UE-RS and the second set of UE-RSmay include 12 UE-RS for a total of 24 UE-RS. The resource block may betransmitted with a set of resource blocks. Moreover, the second set ofUE-RS may include at least one UE-RS when a number of resource blocks inthe set of resource blocks is less than a threshold number.

In an aspect, the same precoding may be used for the first set of UE-RSand the second set of UE-RS. Alternatively, different precoding may beused for the first set of UE-RS and the second set of UE-RS, wherein theother one of the first set of UE-RS and the second set of UE-RS is meantfor the other UE.

In a further aspect, the antenna ports 7, 8, 11, and 13 provide amapping of UE-RS to the first set of UE-RS, wherein the first set ofUE-RS is meant for the UE 2050 and the second set of UE-RS is meant forno other UEs. Accordingly, the resource block processing module 2008 mayinclude the second set of UE-RS in the resource block to enable the UE2050 to perform time tracking based on both the first set of UE-RS andthe second set of UE-RS.

In another aspect, the antenna ports 9, 10, 12, and 14 provide a mappingof UE-RS to the second set of UE-RS, wherein the second set of UE-RS ismeant for the UE 2050 and the first set of UE-RS is meant for no otherUEs. Accordingly, the resource block processing module 2008 may includethe first set of UE-RS in the resource block to enable the UE 2050 toperform time tracking based on both the first set of UE-RS and thesecond set of UE-RS.

The reference signal processing module 2016 configures the UE 2050 toreceive a first number of channel state information reference signal(CSI-RS) ports. The resource block processing module 2008 transmits tothe UE 2050 a resource block comprising a second number of CSI-RS portsgreater than the first number of CSI-RS ports. The second number ofCSI-RS ports enables improved time tracking by the UE 2050. In anaspect, the first number of CSI-RS ports corresponds to a first set ofCSI-RS ports, additional CSI-RS ports in the second number of CSI-RSports correspond to a second set of CSI-RS ports, and the same precodingis applied to the first set of CSI-RS ports and the second set of CSI-RSports in the transmitted resource block.

The proximity information module 2018 may determine proximityinformation via signals received through the receiving module 2004 andtransmits the proximity information to the UE 2050. The proximityinformation may include one of a distance between the UE 2050 and theapparatus 2002 and a distance between the UE 2050 and another eNB, apropagation time from the apparatus 2002 to the UE 2050 and from theother eNB to the UE 2050, a relationship between the distance from theapparatus 2002 and the other eNB, or a relationship between thepropagation time from the apparatus 2002 and the other eNB.

In an aspect, the first number of CSI-RS ports may correspond to a firstconfiguration and additional CSI-RS ports in the second number of CSI-RSports as compared to the first number of CSI-RS ports may correspond toa second configuration. Accordingly, the resource block processingmodule 2008 may transmit a second resource block comprising CSI-RS portscorresponding to only the first configuration.

In an aspect, the rank configuration module 2012 and/or the modulationorder determining module 2006 configures a user equipment (UE) 2050 toreceive a transmission. This may include the transmission module 2020transmitting to the UE 2050 a configuration to receive the transmissionusing a transmission mode supporting cooperative multipoint (CoMP)transmission.

The resource block processing module 2008 transmits to the UE 2050 (viathe transmission module 2020) at least one resource block in thetransmission. The at least one resource block includes a first set ofreference signals (RS), specific to the UE 2050. The resource blockprocessing module 2008 also provides a second set of RS in thetransmission.

The reference signal processing module 2016 may transmit to the UE 2050(via the transmission module 2020) information indicating whether or notthe second set of RS is available to the UE 2050. The proximityinformation module 2018 may also transmit to the UE 2050 proximityinformation. The proximity information includes a propagation timedifference of the first set of RS and the second set of RS. Accordingly,the UE 2050 may perform channel estimation and/or timing estimationbased on the first set of RS and further based on the second set of RSif the second set of RS is available to the UE 2050.

In an aspect, the first set of RS and the second set of RS areUE-specific RS (UE-RS) for demodulation. The first set of UE-RS and thesecond set of UE-RS may have a different precoding or the sameprecoding. The second set of UE-RS may be available to the UE 2050 whenthe second set of UE-RS is contained within the at least one resourceblock. Alternatively, the second set of UE-RS may be available to the UE2050 when a resource block comprising the second set of UE-RS isassociated with a same precoding resource block group (PRG) as the atleast one resource block. In another aspect, the transmission may be forthe UE 2050, the first set of UE-RS may be for the UE 2050, and thesecond set of UE-RS may be for another UE or no other UEs.

In a further aspect, the first set of RS and the second set of RS arechannel state information reference signals (CSI-RS). The first set ofCSI-RS and the second set of CSI-RS may be associated with differentresources. Alternatively, the first set of CSI-RS and the second set ofCSI-RS may be associated with a same set of resources but with differentantenna ports. Moreover, the same precoding may be applied to a firstset of CSI-RS ports and a second set of CSI-RS ports in a transmittedresource block.

In yet another aspect, the first set of RS includes UE-specific RS(UE-RS) for demodulation and the second set of RS includes channel stateinformation reference signals (CSI-RS).

The apparatus may include additional modules that perform each of thesteps of the algorithm in the aforementioned flow charts of FIGS. 11,14, 16, and 19. As such, each step in the aforementioned flow charts ofFIGS. 11, 14, 16, and 19 may be performed by a module and the apparatusmay include one or more of those modules. The modules may be one or morehardware components specifically configured to carry out the statedprocesses/algorithm, implemented by a processor configured to performthe stated processes/algorithm, stored within a computer-readable mediumfor implementation by a processor, or some combination thereof.

FIG. 21 is a conceptual data flow diagram 2100 illustrating the dataflow between different modules/means/components in an exemplaryapparatus 2102. The apparatus may be a UE. The apparatus includes areceiving module 2104, a resource block processing module 2106, areference signal processing module 2108, a channel/timing estimationmodule 2110, a port processing module 2112, a proximity informationmodule 2114, and a transmission module 2116.

In an aspect, the receiving module 2104 may receive a configuration toreceive a transmission using a transmission mode supporting cooperativemultipoint (CoMP) transmission. Accordingly, the resource blockprocessing module 2106 may receive (via the receiving module 2104) aplurality of resource blocks in a transmission. The plurality ofresource blocks may include a precoding resource block group (PRG). Thereference signal processing module 2108 may then decode user equipmentspecific reference signals (UE-RS) based on an assumed same precodingfor transmission of the resource blocks in the PRG. Thereafter, thechannel/timing estimation module 2110 performs time tracking based onthe UE-RS in the PRG decoded by the reference signal processing module2108.

In another aspect, the resource block processing module 2106 may receiveat least one resource block in a transmission. The transmission may be arank one transmission or a rank two transmission. Moreover, each of theat least one resource block may include a first set of user equipmentspecific reference signals (UE-RS). The reference signal processingmodule 2108 may determine whether a resource block of the at least oneresource block includes a second set of UE-RS. The determination ofwhether the resource block of the at least one resource block includesthe second set of UE-RS may be performed only when the at least oneresource block comprises only the resource block. Alternatively, thedetermination may include performing blind detection to determinewhether the resource block includes the second set of UE-RS. In anotheralternative, the determination may include receiving from an eNB 2150information indicating whether the resource block includes the secondset of UE-RS. The channel/timing estimation module 2110 may perform timetracking based on the first set of UE-RS and based on the second set ofUE-RS when the resource block is determined to include the second set ofUE-RS.

In an aspect, the transmission may be meant for the apparatus 2102, thefirst set of UE-RS may be meant for the apparatus 2102, and the secondset of UE-RS may be meant for another UE or no other UEs. Accordingly,the first set of UE-RS and the second set of UE-RS may have a differentprecoding. Alternatively, the first set of UE-RS and the second set ofUE-RS may have the same precoding.

In a further aspect, the receiving module 2104 may receive aconfiguration to receive at least one resource block with a first numberof channel state information reference signal (CSI-RS) ports in eachresource block of the at least one resource block. Accordingly, theresource block processing module 2106 may receive (via the receivingmodule 2104) the at least one resource block in a transmission. The portprocessing module 2112 may assume a resource block of the at least oneresource block includes a second number of CSI-RS ports greater than thefirst number of CSI-RS ports. Specifically, the port processing module2112 may assume the resource block to comprise the second number ofCSI-RS ports when the at least one resource block comprises less than athreshold number of resource blocks and the first number of CSI-RS portsis less than a threshold number of CSI-RS ports.

The port processing module 2112 also determines whether the secondnumber of CSI-RS ports comprises at least a first set of CSI-RS portstransmitted by a first eNB (e.g., eNB 2150) and a second set of CSI-RSports transmitted by a second eNB. Based on a negative outcomedetermined by the port processing module 2112, the channel/timingestimation module 2110 performs time tracking based on signals inresource elements corresponding to the assumed second number of CSI-RSports. However, based on a positive outcome determined by the portprocessing module 2112, the proximity information module 2114 receivesproximity information from a serving eNB (e.g., eNB 2150). The proximityinformation may indicate one of a distance from each of the first eNBand the second eNB, a propagation time from each of the first eNB andthe second eNB, a relationship between the distance from the first eNBand the second eNB, or a relationship between the propagation time fromthe first eNB and the second eNB. Thereafter, the channel/timingestimation module 2110 performs the time tracking further based on thereceived proximity information.

In another aspect, the reference signal processing module 2108 receives(via the receiving module 2104) user equipment specific referencesignals (UE-RS) and channel state information reference signals (CSI-RS)in at least one resource block. Thereafter, the channel/timingestimation module 2110 may perform time tracking based on the receivedUE-RS and CSI-RS.

In a further aspect, the receiving module 2104 may receive aconfiguration to receive a transmission using a transmission modesupporting cooperative multipoint (CoMP) transmission. The resourceblock processing module 2106 receives (via the receiving module 2104) atleast one resource block in the transmission. The at least one resourceblock may include a first set of reference signals (RS), specific to theapparatus 2102.

The reference signal processing module 2108 determines whether a secondset of RS, specific to the apparatus 2102, is available in thetransmission. The determining operation may include the reference signalprocessing module 2108 performing blind detection to determine theavailability of the second set of RS. Alternatively, the determiningoperation may include the reference signal processing module 2108receiving from an evolved Node B (eNB) 2150 information indicatingwhether the second set of RS is available or not.

The proximity information module 2114 may receive proximity informationfrom the eNB 2150. The proximity information may include a propagationtime difference of the first set of RS and the second set of RS. Thechannel/timing estimation module 2110 processes the received at leastone resource block based on the first set of RS and further based on thesecond set of RS if the second set of RS is determined to be available.The processing may include performing channel estimation and/or timingestimation.

In an aspect, the first set of RS and the second set of RS areUE-specific RS (UE-RS) for demodulation. The first set of UE-RS and thesecond set of UE-RS may have a different precoding or the sameprecoding. The availability of the second set of UE-RS may be determinedwhen the second set of UE-RS is contained within the at least oneresource block. Alternatively, the availability of the second set ofUE-RS may be determined when a resource block comprising the second setof UE-RS is associated with a same precoding resource block group (PRG)as the at least one resource block. In another aspect, the transmissionmay be for the apparatus 2102, the first set of UE-RS may be for theapparatus 2102, and the second set of UE-RS may be for another UE or noother UEs.

In a further aspect, the first set of RS and the second set of RS arechannel state information reference signals (CSI-RS). The first set ofCSI-RS and the second set of CSI-RS may be associated with differentresources. Alternatively, the first set of CSI-RS and the second set ofCSI-RS may be associated with a same set of resources but with differentantenna ports. Moreover, the same precoding may be applied to a firstset of CSI-RS ports and a second set of CSI-RS ports in a transmittedresource block.

In yet another aspect, the first set of RS may include UE-specific RS(UE-RS) for demodulation and the second set of RS may include channelstate information reference signals (CSI-RS).

The apparatus may include additional modules that perform each of thesteps of the algorithm in the aforementioned flow charts of FIGS. 12,13, 15, 17, and 18. As such, each step in the aforementioned flow chartsof FIGS. 12, 13, 15, 17, and 28 may be performed by a module and theapparatus may include one or more of those modules. The modules may beone or more hardware components specifically configured to carry out thestated processes/algorithm, implemented by a processor configured toperform the stated processes/algorithm, stored within acomputer-readable medium for implementation by a processor, or somecombination thereof.

FIG. 22 is a diagram 2200 illustrating an example of a hardwareimplementation for an apparatus 2002′ employing a processing system2214. The processing system 2214 may be implemented with a busarchitecture, represented generally by the bus 2224. The bus 2224 mayinclude any number of interconnecting buses and bridges depending on thespecific application of the processing system 2214 and the overalldesign constraints. The bus 2224 links together various circuitsincluding one or more processors and/or hardware modules, represented bythe processor 2204, the modules 2004, 2006, 2008, 2010, 2012, 2014,2016, 2018, 2020, and the computer-readable medium 2206. The bus 2224may also link various other circuits such as timing sources,peripherals, voltage regulators, and power management circuits, whichare well known in the art, and therefore, will not be described anyfurther.

The processing system 2214 may be coupled to a transceiver 2210. Thetransceiver 2210 is coupled to one or more antennas 2220. Thetransceiver 2210 provides a means for communicating with various otherapparatus over a transmission medium. The transceiver 2210 receives asignal from the one or more antennas 2220, extracts information from thereceived signal, and provides the extracted information to theprocessing system 2214, specifically the receiving module 2004. Inaddition, the transceiver 2210 receives information from the processingsystem 2214, specifically the transmission module 2020, and based on thereceived information, generates a signal to be applied to the one ormore antennas 2220. The processing system 2214 includes a processor 2204coupled to a computer-readable medium 2206. The processor 2204 isresponsible for general processing, including the execution of softwarestored on the computer-readable medium 2206. The software, when executedby the processor 2204, causes the processing system 2214 to perform thevarious functions described supra for any particular apparatus. Thecomputer-readable medium 2206 may also be used for storing data that ismanipulated by the processor 2204 when executing software. Theprocessing system further includes at least one of the modules 2004,2006, 2008, 2010, 2012, 2014, 2016, 2018, and 2020. The modules may besoftware modules running in the processor 2204, resident/stored in thecomputer readable medium 2206, one or more hardware modules coupled tothe processor 2204, or some combination thereof. The processing system2214 may be a component of the eNB 610 and may include the memory 676and/or at least one of the TX processor 616, the RX processor 670, andthe controller/processor 675.

In one configuration, the apparatus 2002/2002′ for wirelesscommunication includes means for restricting a number of resource blocksthat can be allocated to a user equipment (UE) in a downlink assignmentto be greater than or equal to N, where N is greater than one, means fortransmitting a downlink transmission corresponding to the downlinkassignment to the UE, means for determining a modulation order for thedownlink transmission, wherein the means for restricting restricts thenumber of resource blocks based on the determined modulation order,means for configuring a user equipment (UE) to receive one of a rank onetransmission or a rank two transmission, means for transmitting aresource block to the UE, the resource block comprising a first set ofuser equipment specific reference signals (UE-RS) and a second set ofUE-RS, one of the first set of UE-RS and the second set of UE-RS beingfor the UE, the other one of the first set of UE-RS and the second setof UE-RS being for another UE or no other UEs, means for using the sameprecoding for the first set of UE-RS and the second set of UE-RS, meansfor using a different precoding for the first set of UE-RS and thesecond set of UE-RS, wherein said other one of the first set of UE-RSand the second set of UE-RS is for said another UE, means fordetermining to transmit the resource block with four antenna ports,means for selecting the four antenna ports to comprise antenna ports 7,8, 11, and 13, wherein the antenna ports 7, 8, 11, and 13 provide amapping of UE-RS to the first set of UE-RS, the first set of UE-RS beingfor the UE and the second set of UE-RS being for no other UEs, means forincluding the second set of UE-RS in the resource block to enable the UEto perform time tracking based on both the first set of UE-RS and thesecond set of UE-RS, means for selecting the four antenna ports tocomprise antenna ports 9, 10, 12, and 14, wherein the antenna ports 9,10, 12, and 14 provide a mapping of UE-RS to the second set of UE-RS,the second set of UE-RS being for the UE and the first set of UE-RSbeing for no other UEs, means for including the first set of UE-RS inthe resource block to enable the UE to perform time tracking based onboth the first set of UE-RS and the second set of UE-RS, means forconfiguring a user equipment (UE) to receive a first number of channelstate information reference signal (CSI-RS) ports, means fortransmitting to the UE a resource block comprising a second number ofCSI-RS ports greater than the first number of CSI-RS ports, the secondnumber of CSI-RS ports enabling improved time tracking by the UE, meansfor transmitting proximity information to the UE, the proximityinformation comprising one of a distance between the UE and the eNB anda distance between the UE and a second eNB, a propagation time from theeNB to the UE and from the second eNB to the UE, a relationship betweenthe distance from the eNB and the second eNB, or a relationship betweenthe propagation time from the eNB and the second eNB, means fortransmitting a second resource block comprising CSI-RS portscorresponding to only a first configuration, wherein the first number ofCSI-RS ports corresponds to the first configuration, and additionalCSI-RS ports in the second number of CSI-RS ports as compared to thefirst number of CSI-RS ports corresponds to a second configuration,means for configuring a user equipment (UE) to receive a transmission,means for transmitting to the UE at least one resource block in thetransmission, where the at least one resource block comprises a firstset of reference signals (RS), specific to the UE, means for providing asecond set of RS in the transmission, means for transmitting to the UEinformation indicating whether or not the second set of RS is availableto the UE, means for transmitting to the UE proximity information,wherein the proximity information comprises a propagation timedifference of the first set of RS and the second set of RS, and meansfor transmitting to the UE a configuration to receive the transmissionusing a transmission mode supporting cooperative multipoint (CoMP)transmission.

The aforementioned means may be one or more of the aforementionedmodules of the apparatus 2002 and/or the processing system 2214 of theapparatus 2002′ configured to perform the functions recited by theaforementioned means. As described supra, the processing system 2214 mayinclude the TX Processor 616, the RX Processor 670, and thecontroller/processor 675. As such, in one configuration, theaforementioned means may be the TX Processor 616, the RX Processor 670,and the controller/processor 675 configured to perform the functionsrecited by the aforementioned means.

FIG. 23 is a diagram 2300 illustrating an example of a hardwareimplementation for an apparatus 2102′ employing a processing system2314. The processing system 2314 may be implemented with a busarchitecture, represented generally by the bus 2324. The bus 2324 mayinclude any number of interconnecting buses and bridges depending on thespecific application of the processing system 2314 and the overalldesign constraints. The bus 2324 links together various circuitsincluding one or more processors and/or hardware modules, represented bythe processor 2304, the modules 2104, 2106, 2108, 2110, 2112, 2114,2116, and the computer-readable medium 2306. The bus 2324 may also linkvarious other circuits such as timing sources, peripherals, voltageregulators, and power management circuits, which are well known in theart, and therefore, will not be described any further.

The processing system 2314 may be coupled to a transceiver 2310. Thetransceiver 2310 is coupled to one or more antennas 2320. Thetransceiver 2310 provides a means for communicating with various otherapparatus over a transmission medium. The transceiver 2310 receives asignal from the one or more antennas 2320, extracts information from thereceived signal, and provides the extracted information to theprocessing system 2314, specifically the receiving module 2104. Inaddition, the transceiver 2310 receives information from the processingsystem 2314, specifically the transmission module 2116, and based on thereceived information, generates a signal to be applied to the one ormore antennas 2320. The processing system 2314 includes a processor 2304coupled to a computer-readable medium 2306. The processor 2304 isresponsible for general processing, including the execution of softwarestored on the computer-readable medium 2306. The software, when executedby the processor 2304, causes the processing system 2314 to perform thevarious functions described supra for any particular apparatus. Thecomputer-readable medium 2306 may also be used for storing data that ismanipulated by the processor 2304 when executing software. Theprocessing system further includes at least one of the modules 2104,2106, 2108, 2110, 2112, 2114, and 2116. The modules may be softwaremodules running in the processor 2304, resident/stored in the computerreadable medium 2306, one or more hardware modules coupled to theprocessor 2304, or some combination thereof. The processing system 2314may be a component of the UE 650 and may include the memory 660 and/orat least one of the TX processor 668, the RX processor 656, and thecontroller/processor 659.

In one configuration, the apparatus 2102/2102′ for wirelesscommunication includes means for receiving a plurality of resourceblocks in a transmission, the plurality of resource blocks comprising aprecoding resource block group (PRG), means for decoding user equipmentspecific reference signals (UE-RS) based on an assumed same precodingfor transmission of the resource blocks in the PRG, means for performingtime tracking based on the decoded UE-RS in the PRG, means for receivinga configuration to receive the transmission using a transmission modesupporting cooperative multipoint (CoMP) transmission, means forreceiving at least one resource block in a transmission, each of said atleast one resource block comprising a first set of user equipmentspecific reference signals (UE-RS), means for determining whether aresource block of the at least one resource block comprises a second setof UE-RS, means for performing time tracking based on the first set ofUE-RS and based on the second set of UE-RS when the resource block isdetermined to comprise the second set of UE-RS, means for receiving aconfiguration to receive at least one resource block with a first numberof channel state information reference signal (CSI-RS) ports in eachresource block of the at least one resource block, means for receivingthe at least one resource block in a transmission, means for assuming aresource block of the at least one resource block comprises a secondnumber of CSI-RS ports greater than the first number of CSI-RS ports,means for performing time tracking based on signals in resource elementscorresponding to the assumed second number of CSI-RS ports, wherein thesecond number of CSI-RS ports comprise at least a first set of CSI-RSports transmitted by a first evolved Node B (eNB) and a second set ofCSI-RS ports transmitted by a second eNB, means for receiving proximityinformation from a serving eNB indicating one of a distance from each ofthe first eNB and the second eNB, a propagation time from each of thefirst eNB and the second eNB, a relationship between the distance fromthe first eNB and the second eNB, or a relationship between thepropagation time from the first eNB and the second eNB, wherein the timetracking is performed further based on the received proximityinformation, means for receiving user equipment specific referencesignals (UE-RS) and channel state information reference signals (CSI-RS)in at least one resource block, means for performing time tracking basedon the received UE-RS and CSI-RS, means for receiving at least oneresource block in a transmission, where the at least one resource blockcomprises a first set of reference signals (RS), specific to the UE,means for determining whether a second set of RS, specific to the UE, isavailable in the transmission, means for processing the received atleast one resource block based on the first set of RS and further basedon the second set of RS if determined available, means for receivingproximity information from an evolved Node B (eNB), wherein theproximity information comprises a propagation time difference of thefirst set of RS and the second set of RS, and means for receiving aconfiguration to receive the transmission using a transmission modesupporting cooperative multipoint (CoMP) transmission.

The aforementioned means may be one or more of the aforementionedmodules of the apparatus 2102 and/or the processing system 2314 of theapparatus 2102′ configured to perform the functions recited by theaforementioned means. As described supra, the processing system 2314 mayinclude the TX Processor 668, the RX Processor 656, and thecontroller/processor 659. As such, in one configuration, theaforementioned means may be the TX Processor 668, the RX Processor 656,and the controller/processor 659 configured to perform the functionsrecited by the aforementioned means.

It is understood that the specific order or hierarchy of steps in theprocesses disclosed is an illustration of exemplary approaches. Basedupon design preferences, it is understood that the specific order orhierarchy of steps in the processes may be rearranged. Further, somesteps may be combined or omitted. The accompanying method claims presentelements of the various steps in a sample order, and are not meant to belimited to the specific order or hierarchy presented.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but is to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. All structural andfunctional equivalents to the elements of the various aspects describedthroughout this disclosure that are known or later come to be known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the claims. Moreover,nothing disclosed herein is intended to be dedicated to the publicregardless of whether such disclosure is explicitly recited in theclaims. No claim element is to be construed as a means plus functionunless the element is expressly recited using the phrase “means for.”.

What is claimed is:
 1. A method of wireless communication of a userequipment (UE), comprising: receiving at least one resource block in atransmission, where the at least one resource block comprises a firstset of reference signals (RS), specific to the UE; determining whether asecond set of RS, specific to the UE, is available in the transmission,wherein the determining comprises receiving from an evolved Node B (eNB)information indicating whether the second set of RS is available or not;and processing the received at least one resource block based on thefirst set of RS and further based on the second set of RS if determinedavailable, wherein if it is determined that the second set of RS isavailable the processing comprises performing time tracking to update adownlink timing of a subframe based at least in part on the second setof RS.
 2. The method of claim 1, wherein the processing comprisesperforming at least one of channel estimation or timing estimation. 3.The method of claim 1, wherein the first set of RS and the second set ofRS are UE-specific RS (UE-RS) for demodulation.
 4. The method of claim3, wherein the availability of the second set of UE-RS is determinedwhen the second set of UE-RS is contained within the at least oneresource block.
 5. The method of claim 3, wherein the availability ofthe second set of UE-RS is determined when a resource block comprisingthe second set of UE-RS is associated with a same precoding resourceblock group (PRG) as the at least one resource block.
 6. The method ofclaim 3, wherein the transmission is for the UE, the first set of UE-RSis for the UE, and the second set of UE-RS is for another UE or no otherUEs.
 7. The method of claim 3, wherein the first set of UE-RS and thesecond set of UE-RS have a different precoding.
 8. The method of claim3, wherein the first set of UE-RS and the second set of UE-RS have thesame precoding.
 9. The method of claim 1, wherein the first set of RSand the second set of RS are channel state information reference signals(CSI-RS).
 10. The method of claim 9, wherein the first set of CSI-RS andthe second set of CSI-RS are associated with different resources. 11.The method of claim 9, wherein the first set of CSI-RS and the secondset of CSI-RS are associated with a same set of resources but withdifferent antenna ports.
 12. The method of claim 9, wherein the sameprecoding is applied to a first set of CSI-RS ports and a second set ofCSI-RS ports in a transmitted resource block.
 13. The method of claim 1,wherein the first set of RS comprises UE-specific RS (UE-RS) fordemodulation and the second set of RS comprises channel stateinformation reference signals (CSI-RS).
 14. The method of claim 1,further comprising receiving proximity information from an evolved NodeB (eNB), wherein the proximity information comprises a propagation timedifference of the first set of RS and the second set of RS.
 15. Themethod of claim 1, further comprising receiving a configuration toreceive the transmission using a transmission mode supportingcooperative multipoint (CoMP) transmission.
 16. A method of wirelesscommunication of an evolved Node B (eNB), comprising: configuring a userequipment (UE) to receive a transmission; transmitting to the UE atleast one resource block in the transmission, where the at least oneresource block comprises a first set of reference signals (RS), specificto the UE; providing a second set of RS in the transmission; andtransmitting to the UE information indicating whether or not the secondset of RS is available to the UE, wherein the second set of RS isconfigured for use in performing time tracking to update a downlinktiming of a subframe.
 17. The method of claim 16, wherein the UEperforms at least one of channel estimation or timing estimation basedon the first set of RS and further based on the second set of RS if thesecond set of RS is available to the UE.
 18. The method of claim 17,wherein the first set of RS and the second set of RS are UE-specific RS(UE-RS) for demodulation.
 19. The method of claim 18, wherein the secondset of UE-RS is available to the UE when the second set of UE-RS iscontained within the at least one resource block.
 20. The method ofclaim 18, wherein the second set of UE-RS is available to the UE when aresource block comprising the second set of UE-RS is associated with asame precoding resource block group (PRG) as the at least one resourceblock.
 21. The method of claim 18, wherein the transmission is for theUE, the first set of UE-RS is for the UE, and the second set of UE-RS isfor another UE or no other UEs.
 22. The method of claim 18, wherein thefirst set of UE-RS and the second set of UE-RS have a differentprecoding.
 23. The method of claim 18, wherein the first set of UE-RSand the second set of UE-RS have the same precoding.
 24. The method ofclaim 17, wherein the first set of RS and the second set of RS arechannel state information reference signals (CSI-RS).
 25. The method ofclaim 24, wherein the first set of CSI-RS and the second set of CSI-RSare associated with different resources.
 26. The method of claim 24,wherein the first set of CSI-RS and the second set of CSI-RS areassociated with a same set of resources but with different antennaports.
 27. The method of claim 24, wherein the same precoding is appliedto a first set of CSI-RS ports and a second set of CSI-RS ports in atransmitted resource block.
 28. The method of claim 17, wherein thefirst set of RS comprises UE-specific RS (UE-RS) for demodulation andthe second set of RS comprises channel state information referencesignals (CSI-RS).
 29. The method of claim 16, further comprisingtransmitting to the UE proximity information, wherein the proximityinformation comprises a propagation time difference of the first set ofRS and the second set of RS.
 30. The method of claim 16, furthercomprising transmitting to the UE a configuration to receive thetransmission using a transmission mode supporting cooperative multipoint(CoMP) transmission.
 31. A user equipment (UE), comprising: means forreceiving at least one resource block in a transmission, where the atleast one resource block comprises a first set of reference signals(RS), specific to the UE; means for determining whether a second set ofRS, specific to the UE, is available in the transmission, wherein themeans for determining is configured to receive from an evolved Node B(eNB) information indicating whether the second set of RS is availableor not; and means for processing the received at least one resourceblock based on the first set of RS and further based on the second setof RS if determined available, wherein if it is determined that thesecond set of RS is available the means for processing is configured toperform time tracking to update a downlink timing of a subframe based atleast in part on the second set of RS.
 32. The UE of claim 31, whereinthe means for processing is configured to perform at least one ofchannel estimation or timing estimation.
 33. The UE of claim 31, whereinthe first set of RS and the second set of RS are UE-specific RS (UE-RS)for demodulation.
 34. The UE of claim 33, wherein the availability ofthe second set of UE-RS is determined when the second set of UE-RS iscontained within the at least one resource block.
 35. The UE of claim33, wherein the availability of the second set of UE-RS is determinedwhen a resource block comprising the second set of UE-RS is associatedwith a same precoding resource block group (PRG) as the at least oneresource block.
 36. The UE of claim 33, wherein the transmission is forthe UE, the first set of UE-RS is for the UE, and the second set ofUE-RS is for another UE or no other UEs.
 37. The UE of claim 33, whereinthe first set of UE-RS and the second set of UE-RS have a differentprecoding.
 38. The UE of claim 33, wherein the first set of UE-RS andthe second set of UE-RS have the same precoding.
 39. The UE of claim 31,wherein the first set of RS and the second set of RS are channel stateinformation reference signals (CSI-RS).
 40. The UE of claim 39, whereinthe first set of CSI-RS and the second set of CSI-RS are associated withdifferent resources.
 41. The UE of claim 39, wherein the first set ofCSI-RS and the second set of CSI-RS are associated with a same set ofresources but with different antenna ports.
 42. The UE of claim 39,wherein the same precoding is applied to a first set of CSI-RS ports anda second set of CSI-RS ports in a transmitted resource block.
 43. The UEof claim 31, wherein the first set of RS comprises UE-specific RS(UE-RS) for demodulation and the second set of RS comprises channelstate information reference signals (CSI-RS).
 44. The UE of claim 31,further comprising means for receiving proximity information from anevolved Node B (eNB), wherein the proximity information comprises apropagation time difference of the first set of RS and the second set ofRS.
 45. The UE of claim 31, further comprising means for receiving aconfiguration to receive the transmission using a transmission modesupporting cooperative multipoint (CoMP) transmission.
 46. An evolvedNode B (eNB), comprising: means for configuring a user equipment (UE) toreceive a transmission; means for transmitting to the UE at least oneresource block in the transmission, where the at least one resourceblock comprises a first set of reference signals (RS), specific to theUE; means for providing a second set of RS in the transmission; andmeans for transmitting to the UE information indicating whether or notthe second set of RS is available to the UE, wherein the second set ofRS is configured for use in performing time tracking to update adownlink timing of a subframe.
 47. The eNB of claim 46, wherein the UEperforms at least one of channel estimation or timing estimation basedon the first set of RS and further based on the second set of RS if thesecond set of RS is available to the UE.
 48. The eNB of claim 47,wherein the first set of RS and the second set of RS are UE-specific RS(UE-RS) for demodulation.
 49. The eNB of claim 48, wherein the secondset of UE-RS is available to the UE when the second set of UE-RS iscontained within the at least one resource block.
 50. The eNB of claim48, wherein the second set of UE-RS is available to the UE when aresource block comprising the second set of UE-RS is associated with asame precoding resource block group (PRG) as the at least one resourceblock.
 51. The eNB of claim 48, wherein the transmission is for the UE,the first set of UE-RS is for the UE, and the second set of UE-RS is foranother UE or no other UEs.
 52. The eNB of claim 48, wherein the firstset of UE-RS and the second set of UE-RS have a different precoding. 53.The eNB of claim 48, wherein the first set of UE-RS and the second setof UE-RS have the same precoding.
 54. The eNB of claim 47, wherein thefirst set of RS and the second set of RS are channel state informationreference signals (CSI-RS).
 55. The eNB of claim 54, wherein the firstset of CSI-RS and the second set of CSI-RS are associated with differentresources.
 56. The eNB of claim 54, wherein the first set of CSI-RS andthe second set of CSI-RS are associated with a same set of resources butwith different antenna ports.
 57. The eNB of claim 54, wherein the sameprecoding is applied to a first set of CSI-RS ports and a second set ofCSI-RS ports in a transmitted resource block.
 58. The eNB of claim 47,wherein the first set of RS comprises UE-specific RS (UE-RS) fordemodulation and the second set of RS comprises channel stateinformation reference signals (CSI-RS).
 59. The eNB of claim 46, furthercomprising means for transmitting to the UE proximity information,wherein the proximity information comprises a propagation timedifference of the first set of RS and the second set of RS.
 60. The eNBof claim 46, further comprising means for transmitting to the UE aconfiguration to receive the transmission using a transmission modesupporting cooperative multipoint (CoMP) transmission.
 61. A userequipment (UE), comprising: a memory; and a processing system coupled tothe memory and configured to: receive at least one resource block in atransmission, where the at least one resource block comprises a firstset of reference signals (RS), specific to the UE; determine whether asecond set of RS, specific to the UE, is available in the transmission,wherein the processing system configured to determine is furtherconfigured to receive from an evolved Node B (eNB) informationindicating whether the second set of RS is available or not; and processthe received at least one resource block based on the first set of RSand further based on the second set of RS if determined available,wherein if it is determined that the second set of RS is available theprocessing system is configured to process the at least one resourceblock by performing time tracking to update a downlink timing of asubframe based at least in part on the second set of RS.
 62. The UE ofclaim 61, wherein the processing system configured to process is furtherconfigured to perform at least one of channel estimation or timingestimation.
 63. The UE of claim 61, wherein the first set of RS and thesecond set of RS are UE-specific RS (UE-RS) for demodulation.
 64. The UEof claim 63, wherein the availability of the second set of UE-RS isdetermined when the second set of UE-RS is contained within the at leastone resource block.
 65. The UE of claim 63, wherein the availability ofthe second set of UE-RS is determined when a resource block comprisingthe second set of UE-RS is associated with a same precoding resourceblock group (PRG) as the at least one resource block.
 66. The UE ofclaim 63, wherein the transmission is for the UE, the first set of UE-RSis for the UE, and the second set of UE-RS is for another UE or no otherUEs.
 67. The UE of claim 63, wherein the first set of UE-RS and thesecond set of UE-RS have a different precoding.
 68. The UE of claim 63,wherein the first set of UE-RS and the second set of UE-RS have the sameprecoding.
 69. The UE of claim 61, wherein the first set of RS and thesecond set of RS are channel state information reference signals(CSI-RS).
 70. The UE of claim 69, wherein the first set of CSI-RS andthe second set of CSI-RS are associated with different resources. 71.The UE of claim 69, wherein the first set of CSI-RS and the second setof CSI-RS are associated with a same set of resources but with differentantenna ports.
 72. The UE of claim 69, wherein the same precoding isapplied to a first set of CSI-RS ports and a second set of CSI-RS portsin a transmitted resource block.
 73. The UE of claim 61, wherein thefirst set of RS comprises UE-specific RS (UE-RS) for demodulation andthe second set of RS comprises channel state information referencesignals (CSI-RS).
 74. The UE of claim 61, the processing system furtherconfigured to receive proximity information from an evolved Node B(eNB), wherein the proximity information comprises a propagation timedifference of the first set of RS and the second set of RS.
 75. The UEof claim 61, the processing system further configured to receive aconfiguration to receive the transmission using a transmission modesupporting cooperative multipoint (CoMP) transmission.
 76. An evolvedNode B (eNB), comprising: a memory; and a processing system coupled tothe memory and configured to: configure a user equipment (UE) to receivea transmission; transmit to the UE at least one resource block in thetransmission, where the at least one resource block comprises a firstset of reference signals (RS), specific to the UE; provide a second setof RS in the transmission; and transmit to the UE information indicatingwhether or not the second set of RS is available to the UE, wherein thesecond set of RS is configure for use in performing time tracking toupdate a downlink timing of a subframe.
 77. The eNB of claim 76, whereinthe UE performs at least one of channel estimation or timing estimationbased on the first set of RS and further based on the second set of RSif the second set of RS is available to the UE.
 78. The eNB of claim 77,wherein the first set of RS and the second set of RS are UE-specific RS(UE-RS) for demodulation.
 79. The eNB of claim 78, wherein the secondset of UE-RS is available to the UE when the second set of UE-RS iscontained within the at least one resource block.
 80. The eNB of claim78, wherein the second set of UE-RS is available to the UE when aresource block comprising the second set of UE-RS is associated with asame precoding resource block group (PRG) as the at least one resourceblock.
 81. The eNB of claim 78, wherein the transmission is for the UE,the first set of UE-RS is for the UE, and the second set of UE-RS is foranother UE or no other UEs.
 82. The eNB of claim 78, wherein the firstset of UE-RS and the second set of UE-RS have a different precoding. 83.The eNB of claim 78, wherein the first set of UE-RS and the second setof UE-RS have the same precoding.
 84. The eNB of claim 77, wherein thefirst set of RS and the second set of RS are channel state informationreference signals (CSI-RS).
 85. The eNB of claim 84, wherein the firstset of CSI-RS and the second set of CSI-RS are associated with differentresources.
 86. The eNB of claim 84, wherein the first set of CSI-RS andthe second set of CSI-RS are associated with a same set of resources butwith different antenna ports.
 87. The eNB of claim 84, wherein the sameprecoding is applied to a first set of CSI-RS ports and a second set ofCSI-RS ports in a transmitted resource block.
 88. The eNB of claim 77,wherein the first set of RS comprises UE-specific RS (UE-RS) fordemodulation and the second set of RS comprises channel stateinformation reference signals (CSI-RS).
 89. The eNB of claim 76, theprocessing system further configured to transmit to the UE proximityinformation, wherein the proximity information comprises a propagationtime difference of the first set of RS and the second set of RS.
 90. TheeNB of claim 76, the processing system further configured to transmit tothe UE a configuration to receive the transmission using a transmissionmode supporting cooperative multipoint (CoMP) transmission.
 91. Anon-transitory computer-readable medium storing computer executable codefor wireless communication, comprising code for: receiving at least oneresource block in a transmission, where the at least one resource blockcomprises a first set of reference signals (RS), specific to the UE;determining whether a second set of RS, specific to the UE, is availablein the transmission, wherein the determining comprises receiving from anevolved Node B (eNB) information indicating whether the second set of RSis available or not; and processing the received at least one resourceblock based on the first set of RS and further based on the second setof RS if determined available, wherein if it is determined that thesecond set of RS is available the processing comprises performing timetracking to update a downlink timing of a subframe based at least inpart on the second set of RS.
 92. A non-transitory computer-readablemedium storing computer executable code for wireless communication,comprising code for: configuring a user equipment (UE) to receive atransmission; transmitting to the UE at least one resource block in thetransmission, where the at least one resource block comprises a firstset of reference signals (RS), specific to the UE; providing a secondset of RS in the transmission; and transmitting to the UE informationindicating whether or not the second set of RS is available to the UE,wherein the UE performs at least one of channel estimation or timingestimation based on the first set of RS and further based on the secondset of RS if the second set of RS is available to the UE, and whereinthe second set of RS is configured for use in performing time trackingto update a downlink timing of a subframe.
 93. The method of claim 1,wherein the first set of RS and the second set of RS are both receivedin a same resource block.
 94. The method of claim 16, wherein the firstset of RS and the second set of RS are both transmitted in a sameresource block.
 95. The UE of claim 31, wherein the first set of RS andthe second set of RS are both received in a same resource block.
 96. TheeNB of claim 46, wherein the first set of RS and the second set of RSare both transmitted in a same resource block.
 97. The UE of claim 61,wherein the first set of RS and the second set of RS are both receivedin a same resource block.
 98. The eNB of claim 76, wherein the first setof RS and the second set of RS are both transmitted in a same resourceblock.